1. Preheat the oven to 180 degrees. Cream the sugar, eggs and butter in a food processor for five minutes. Sift the flour and baking powder into a bowl, and add to the egg mixture along with the lemon zest. Mix for a further five minutes.
2. Meanwhile, grease the Bundt cake tin. Pour the cake mixture into the tin so that it’s evenly spread, and place in the oven.
3. While the cake is cooking, prepare the first layer of icing: sift 4 tbsp of the icing sugar into a bowl, add a few drops of lemon juice and mix well. Keep adding lemon juice until the icing is smooth in texture (keep a little juice aside for the next batch of icing). After 45-50 minutes, check the cake is cooked by pushing a sharp knife into the sponge — if the blade is clean, it’s done. Remove the cake from the oven and allow to stand for five minutes before turning out onto a wire rack, then allow to cool thoroughly.
4. For the decoration: Drizzle the icing from the outside into the centre of the cake, then sprinkle with lemon zest. Allow to sit while you prepare the yellow icing: add the remainder of the icing sugar to a bowl with the yellow food colouring, mix to a paste, add the remaining drops of lemon juice until the mixture is smooth. Repeat the icing process, spooning from outside to the centre. Leave to set while you melt the chocolate.
5. Place a small pan with a little water over heat and bring to the boil. Break the chocolate squares into a small glass bowl and place over the pan, making sure that the bowl does not touch the water. When the chocolate has melted, remove from the heat and let stand for a few minutes. Using the same technique, drizzle the sponge with the chocolate. Leave to set then serve.
– Recipe and photography by Mark Setchfield