Octonauts Underwater Cake

“Explore! Rescue! Protect! Eat Cake!”


For the cake sponges

  • 8 large eggs
  • 450g caster sugar
  • 450g unsalted butter, room temperature (plus extra for greasing)
  • 450g plain flour
  • 5 tsp baking powder
  • zest of 4 lemons
  • 100ml lemon juice (roughly equivalent to the juice of 2 lemons)

For the submarine sponges

  • 4 large eggs
  • 225g caster sugar
  • 225g unsalted butter, room temperature
  • 225g plain flour
  • 3 tsp baking powder
  • zest of 2 lemons
  • 50ml lemon juice (around 1 lemon’s worth)

For the filling

  • 250g unsalted butter, room temperature
  • 500g icing sugar
  • ½ tsp vanilla paste
  • 200g raspberry jam (no bits)

For the topping

  • 2kg fondant icing
  • Food colouring gels

Kit needed

  • 23cm springform cake tin
  • 1lb (500g) loaf tin
  • Baking parchment
  • Square cake board, mine was 36cm2
  • Electric hand whisk or electric stand mixer
  • Small offset palette knife
  • Rolling pin
  • Cooling rack
  • Cake cutting wire (optional but very useful)
  • Cake smoother (optional but very useful)


Cake sponges

  1. This cake is made from two large sponges cut into two layers each, so there are four layers inside the cake. Make each sponge cake one at a time with 4 large eggs, 225g unsalted butter, 225g caster sugar, 225g plain flour, 2½ tsp baking powder and the zest of 2 lemons (plus the juice of 1) in each cake. You might want to make the sponges in advance, in which case wrap them well in cling film to prevent them drying out.
  2. Preheat the oven to 180oC / 160oC fan / gas mark 4. Take your springform cake tin, line with baking parchment and grease well.
  3. Put the butter and sugar into a bowl and beat with an electric hand whisk or in a stand mixer until light and creamy. Thoroughly beat in each egg, one by one.
  4. Add the lemon zest and juice, sift in the rest of the flour and baking powder and fold them in. Be gentle but make sure the mixture is fully combined, including any flour at the bottom of the bowl. Put this into the cake tin, smooth the batter out with an offset palette knife and bake for 45-50 minutes.
  5. Check that the cake is cooked by poking a cocktail stick into the centre: if it comes out clean then it’s cooked; if it’s got cake mix on it, give the cake another few minutes before testing again.
  6. Take out of the oven and set on a wire rack. Once cool enough to touch, pop the cake out of the tin and return the sponge to the rack to fully cool. Repeat.
  7. When the cakes have cooled, use your cake cutting wire (or a sharp bread knife if you don’t have a cutting wire) to level the tops of the sponges and to cut them through the middle to turn your two large sponges into four thinner layers.

Submarine sponges

  1. The smaller sponges for the submarine have the same ingredients as the cakes above but baked in a loaf tin, greased and lined with baking parchment. Each cake takes a mix of 2 large eggs, 112g unsalted butter, 112g caster sugar, 112g plain flour, 1½ tsp baking powder and the zest of 1 lemon (plus the juice of half a lemon) in each cake.
  2. Make two cakes in the loaf tin as per the instructions above but bake for 35 minutes at 180oC / 160oC fan / gas mark 4.


  1. To make the buttercream, beat the butter in a large bowl with an electric hand whisk or in a stand mixer. Pour in the icing sugar, add the vanilla paste and mix slowly on the lowest setting on your mixer until completely incorporated.
  2. Take the first layer of cake and put onto your cake board. Spread a thin layer of buttercream onto the bottom cake and smooth with an offset spatula. Add a thin layer of jam (around 2tsps smoothed out) on top of the buttercream. Spread a thin layer of buttercream onto the bottom of the next cake layer and lay the next sponge on top of the cake.
  3. Repeat until it is a four layer sponge cake (sponge, buttercream, jam, buttercream, sponge, etc.). Coat the entire cake in a thin layer of buttercream and smooth with an offset palette knife.

Submarine construction

  1. The submarine is made from two pieces of cake. I searched online for ‘Octonauts Gup-A’ and there were loads of images to guide the shape. The bottom cake should be carved carefully with a serrated bread knife into a teardrop shape with a slight curve on the underside. The top cake should be carved into a triangular shape with a slight curve to the top.
  2. Sandwich the two layers together with buttercream and add a light crumb coat of buttercream around both cakes.


  1. Using a small amount of blue gel food colouring, colour 1kg of fondant to cover the cake. Roll this out to a circle of about 40cm diameter and use your rolling pin to carefully lift this onto the cake. Use a cake smoother (or your hands) to smooth the fondant onto and all around the cake. Cut the excess fondant from the base of the cake with a sharp knife.
  2. Colour small amounts of fondant icing with all the colours you will need and wrap each ball in cling film to prevent it from hardening or drying out.
  3. Colour some more fondant in a slightly different shade of blue to cover your submarine. Lay the fondant over the sponges and press down gently around the contours of the sponges. It doesn’t have to completely encase the submarine – tuck the fondant in around the bottom of the sponges. Ensure you leave lots of slack around the cockpit so that the fondant doesn’t split.
  4. With the base cake and the submarine covered in fondant now is your time to get creative with fondant decorations!
  5. For the Gup-A, use fondant to make the heads of Kwazii, Captain Barnacles and Peso. I rested mine in the cockpit but if you want to secure them further, you can place the heads on cocktail sticks.
  6. I wrapped coat hanger wire in fondant for the antenna – if you do this keep an eye on your kids (or grown-ups) to make sure they don’t try and bite into this! I usually avoid using any non edible materials in cakes but here it was unavoidable.
  7. Stick the decorations onto the fondant base with a tiny amount of buttercream. For the fins, cut the triangular shapes out and let the fondant dry and harden a little. Use two cocktail sticks on each side to rest the fins on, as in the picture.
  8. Using curved strips of yellow and green fondant, wrap the base of the cake to give the effect of ocean contours and the sea bed. Attach these to the sides of the cake with a little buttercream.
  9. Use the different coloured fondants to create different under water features – as many or as few as you like but the more colourful the better. Again, stick these to the sides of the cake with a little buttercream. This is a great, creative way to get the kids involved in making the cake too – everyone can have a go at making different pieces of coral, foliage or shells.
  10. Sound the Octo-Alert – it’s birthday cake time!

For more baking tips and tricks, read Richard's blog.

If you're looking for some further inspiration for a little one's birthday or party, why not check out our CBeebies-themed cake board on Pinterest.

Get the rest of Richard's CBeebies cake recipes.

Pro tip:

Step-by-step guide to making your own Peso.

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