The Hairy Bikers give a lemony twist to the traditional Swiss roll.
Grease and line a 33cm x 23cm/13in x 9in Swiss roll tin with buttered baking parchment.
Preheat the oven to 200C/400F/Gas 6.
Put the eggs, 115g/4oz of the sugar and the lemon zest in a heatproof bowl and place over a pan of gently simmering water.
Whisk with an electric whisk until the mixture is pale, creamy and thick enough to leave a trail when the whisk is lifted.
Carefully remove the bowl from the heat and continue whisking for a further five minutes.
Sift in half the flour and, using a large metal spoon, lightly fold it into the egg mixture.
Sift in the remaining flour and fold into the mixture - it’s important to use gentle movements to retain as much air as possible in the batter.
Pour the mixture slowly into the prepared tin and gently spread with a spatula so the base of the tin is evenly covered.
Bake for 10–12 minutes until well-risen, pale golden-brown and firm to the touch.
Place a damp tea towel on the work surface and cover with a sheet of baking parchment.
Dredge with the four tablespoons of sugar – this will help stop the outside of the sponge sticking. Working quickly, turn the cake out onto the sugared paper and carefully remove the lining paper.
Using a sharp knife, cut off the crusty edges from the two long sides. Roll the Swiss roll from one of the short ends, starting with a tight turn to make a good round shape and keeping the sugared paper inside the roll. Set on a wire rack and leave to cool.
To make the icing, put the icing sugar and butter in a bowl and beat until smooth and creamy. Add the lemon juice and beat again.
When the cake is cold, gently unroll but do not flatten it or it could crack.
Spread the cake with then lemon butter icing, leaving a 1cm/½in border.
Spoon the lemon curd evenly over the icing.
Slowly roll the cake up again, enclosing the filling, and place on a plate. Sprinkle with extra sugar if you like.
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