First, prepare the champagne reduction for the buttercream. Put the champagne and caster sugar in a saucepan over a medium–high heat for 10–12 minutes until reduced and syrupy. Transfer to a small bowl and chill in the fridge for a couple of hours.
Preheat the oven to 180C/160C Fan/Gas 4. Grease three 20cm/8in round cake tins and line the bases with baking paper.
To make the vanilla sponge, put the butter and sugar in a freestanding mixer and beat well for a couple of minutes, until pale. Add the eggs, one by one, mixing after each addition. Gradually add the flour and mix again to combine. Add the vanilla extract and milk and mix to combine.
Divide the mixture equally between the prepared tins. Bake for 25–30 minutes, turning the tins after 20 minutes if necessary so they colour evenly. The cakes are done when they are lightly golden and feel springy to the touch. Leave to cool in the tins for 10–15 minutes. Remove from the tins and transfer to a wire rack to cool completely.
To make the buttercream, put the butter and icing sugar in a freestanding mixer and beat for a few minutes, until light and creamy. Add the champagne reduction and mix for another minute or two, until well combined. Add a few drops of pink food colouring to achieve the desired shade of pink, if you like.
To assemble: if necessary, use a large serrated knife to slice off the domed tops of the cakes so they are flat. Put one sponge on a serving plate, cake board or cake stand. Spread a layer of buttercream over the sponge, followed by a generous layer of the raspberry jam. Repeat with another layer of sponge, buttercream and jam and finish with the third sponge on top. Crumb coat the cake by spreading a thin layer of the buttercream all over the sides and top of the cake, using a large palette knife. This will help to seal in the crumbs and make the final coat of buttercream much easier to apply. Chill the cake for 15–20 minutes.
To finish, spread the remaining buttercream all over the cake in an even layer, using a cake smoother or large palette knife to get it super smooth.
To decorate, arrange the ice cream cone in the middle of the cake and fill with salted caramel popcorn and malted milk balls, so they are spilling out and down the side. Decorate with chocolate-coated strawberries, if using.