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1
Grease and line a 33 x 23cm Swiss roll tin with buttered baking parchment.
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2
Preheat the oven to 200C/Gas 6.
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3
Put the eggs, 115g of sugar and lemon zest in a heatproof bowl and place over a pan of gently simmering water.
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4
Whisk with an electric whisk until the mixture is pale, creamy and thick enough to leave a trail when the whisk is lifted.
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5
Carefully remove the bowl from the heat and continue whisking for a further 5 minutes.
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6
Sift over half the flour and, using a large metal spoon, lightly fold it into the egg mixture.
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7
Sift over the remaining flour and fold in.
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8
It's important to use gentle movements to retain as much air as possible in the batter, but you'll need to watch out for pockets of flour.
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9
Pour the mixture slowly into the prepared tin and gently spread with a spatula, so the base of the tin is evenly covered.
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10
Bake for 10-12 minutes until well risen, pale golden brown and firm to the touch.
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11
Place a damp tea towel on the work surface and cover with a sheet of baking parchment.
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12
Dredge with the 4 tbsp of sugar - this will help stop the outside of the sponge sticking.
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13
Working quickly, turn the cake out onto the sugared paper and carefully remove the lining paper.
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14
Using a sharp knife, cut off the crusty edges from the 2 long sides.
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15
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.
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16
Set on a wire rack and leave to cool.
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17
To make the icing, put the icing sugar and butter in a bowl and beat until smooth and creamy.
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18
Add the lemon juice and beat again.
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19
When the cake is cold, gently unroll but do not flatten it or it could crack.
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20
Spread the cake with lemon butter icing to within 1cm of the edges.
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21
Spoon the lemon curd on top - you'll need about half the jar - and spread it evenly over the icing.
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22
Slowly roll the cake up again, enclosing the filling, and place on a plate.
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23
Sprinkle with extra sugar if you like.
