Method
Put margarine, sugar, and lemon zest in a mixing bowl. Mix till light and fluffy, sift flour and baking powder, alternate with the flour and eggs, then fold in remaining flour, divide into tins, bake at Gas Mark 4 / 180c for approx 30mins.
To make the buttercream filling. When cakes have gone cold, beat margarine till pale and creamy, add sieved icing sugar a bit at a time till smooth, add tsp lemon juice from lemon, mix in, put lemon curd on one half of the cakes and smooth buttercream on the other, then sandwich them both together, this is easy to do as the buttercream and lemon curd sticks to the cakes.
To make the topping beat sieved icing sugar with juice from lemon, plus water, to make a stiff paste, it needs to be runny but still quite stiff, place the icing on top of the cake, take a look at pictures for help. Then just nudge it to the sides so it will just gently slide down the sides, if you want to decorate the cake, I add some lemon jellies or swirls of lemon round the top, its up to you to be as creative as you like or just leave as it is it tastes delicious either way.
If you live alone, or don’t want to eat it all in one go this cake freezes well so you could pop some in the freezer to eat whenever you like.
Some pictures to help are below, in step-by-step order.
Step 1