Put the dates into a saucepan with the water, brown sugar, butter and cocoa. Cook gently over a low heat, stirring constantly until the mixture is smooth and paste-like. Add the vanilla essence and set aside to cool.
Preheat the oven to 180˚C. Lightly grease a 20cm square cake tin and line the base and two sides with baking paper.
Cream the butter and sugar until light and fluffy then add the egg and beat well.
Sift together the flour and baking powder and mix in well.
Press half of the dough into the base of the prepared tin and spread with the cooled date mixture.
Pat or roll out the remaining dough on a piece of greaseproof or baking paper. Flip it onto the date filling and press down lightly.
Bake for 30 minutes or until lightly browned. Cool in the tin on a wire rack, dust with icing sugar then turn out of the tin and cut into fingers.