Method

Preheat the oven to 160°C/140°C fan. Grease 2x20cm round sandwich tins and line the bases. Crack the eggs into a large mixing bowl and add the remaining sponge ingredients. Beat with an electric whisk until just incorporated. Divide the mixture between tins and place in the oven to bake for 20-25 minutes or until golden and risen. Insert a skewer and if it comes out clean the cake is baked. If there is cake mixture attached to the skewer, return the cake to the oven for a further 5 minutes. 

Meanwhile, to make the filling, place the butter and icing sugar in a large mixing bowl. Use a clean electric whisk to beat until light and fluffy. Add the lemon zest and quickly beat again. Cover and place to one side in a cool place. 

When the sponges are baked, remove from the oven. Allow to sit in the tins for 5 minutes, then turn out onto a cooling rack. Once the sponges have cooled completely, place 1 sponge onto a serving board or plate. Spread the buttercream icing over the sponge and spoon over the lemon curd. Sandwich the other sponge on top and dust with icing sugar.