Make sure your traybake is still warm when you add the drizzle – that way the syrup will soak into the sponge leaving some of the sugar on the top.
Heat the oven to 170°C/150°C fan/325°F/Gas 3.
Put all the sponge ingredients in the bowl of a stand mixer fitted with the beater attachment. Beat the mixture for 2 minutes on medium speed until smooth.
Transfer the mixture to the prepared traybake tin and smooth the top. Bake for 35–40 minutes, until springy to touch and a skewer inserted into the centre comes out clean. Remove from the oven and set aside while you make the drizzle.
Stir together the lemon juice and sugar. Using a cocktail stick, poke holes all over the sponge and spoon the drizzle all over. Leave to cool in the tin.
Make the decoration. In a medium pan, dissolve the sugar in 100ml of water. Add the lemon slices and simmer over a low heat for 15 minutes to soften the lemon slices. Use a fork lift the slices out of the pan and arrange them in 3 rows of 4 slices over the top of the cake.
Remove the cake from the tin and cut it into squares – with a lemon slice on top of each. Serve.