1. Preheat the oven to 160° C. Grease a cake tin. Sieve the flour along with baking powder into a bowl. Add the walnuts and raisins and mix.
2. Cream the butter and brown sugar with the vanilla essence in another bowl. Add the eggs and lemon zest, mixing continuously. Fold in sieved flour and mix.
3. Pour the cake batter into the greased tin and bake in the preheated oven for fifteen to twenty minutes. Remove from oven and turn onto a rack to cool.
4. Slice and serve.