Heat oven to 180C/fan 160C/gas 4 and make sure there’s a shelf ready in the middle of the oven. Butter a rectangular baking tray or small roasting tin, about 20cm x 30cm. Cut out a sheet of baking paper a bit larger than the tin, then push it in and smooth it out with your hands so it sticks to the butter. Snip into the corners with a pair of scissors to get the paper to lie neatly.
Put all the cake ingredients and a pinch of salt into a large bowl, then use electric beaters to beat until creamy and smooth. Scoop into the tin, then level the top. Spoon the lemon curd over the batter in thick stripes. Use the handle of the spoon to swirl the curd into the cake - not too much or you won’t see the swirls once it’s cooked. Bake for about 35 mins or until golden and risen. It should have shrunk away from the sides of the tin ever so slightly and feel springy. Don’t open the oven before 30 mins cooking is up.
Leave the cake in the tin for 10 mins or until just cool enough to handle. Carefully lift out of the tin and put it onto a cooling rack, sat over a tray or something similar to catch drips of drizzle. To make the drizzle, mix 4 tbsp sugar and the lemon juice together and spoon over the cake. Toss the lemon zest with the final 1 tbsp sugar and scatter over the top. Let the cake cool completely, then lift onto a board, peel away the sides of the baking paper and cut the cake into fingers. Will keep in an airtight tin for 3 days.