Pre-heat your oven to 180°c (160°c for fan assisted ovens of Gas Mark 4) and line a 12-hole muffin tray with fairy cake cases. Set aside.
In a large bowl, cream together the margarine and caster sugar, until it is fluffy in texture and light in colour. Add the lemon extract (if using) and mix through.
Add the eggs one at a time, ensuring each is mixed through before adding the next. If the mixture begins to curdle, add a spoonful of your flour to help it bind.
Sift in the flour and baking powder, and gently fold to create a batter.
Spoon the batter into your prepared fairy cake cases; about a tablespoon into each, filling them about half-full.
Bake your cakes in your pre-heated oven for about 10-15 minutes, until they are risen, golden in colour and a skewer inserted comes out clean.
Once cool enough to touch, transfer to a wire rack to cool completely. Meanwhile, prepare your buttercream.