Cream the butter with the sugar and then beat in the eggs one by one.
Fold in the flour and mix until combined.
Add half the lemon zest and mix gently.
In a separate bowl, mash the blackberries with the back of a fork, add the other half of the lemon zest and a teaspoon of caster sugar to sweeten. Mix.
Lightly spray the cake moulds to prevent sticking then fill each hole about a third of the way up.
Using a teaspoon, add a little of the blackberry puree on top of the cake batter. You won't use it all up and could drizzle the rest over the cakes after baking.
Add more cake batter on top of the blackberry puree, ensuring you don't overfill the holes. They should only be approx two thirds full.
Bake at 180 degrees in a preheated oven for 15-18 minutes or until golden brown and a cake tester comes out clean.
Allow to cool in the tin and then remove gently and leave them to cool fully. Dust with icing sugar and serve.