Preheat the oven to 180 C / Gas 4. Grease a medium baking tin and line with baking parchment.
In a mixing bowl, combine the flour, cocoa powder, baking powder, sugar and Weetabix®. Mix well. Add the melted butter and stir then gradually add the milk, using just enough to keep mixture moist. Transfer to the prepared tin and press down to compact the mixture.
Bake in the preheated oven for 20 to 25 minutes until firm. Remove from the oven and allow to cool to room temperature, about 40 minutes then place in the fridge to chill, about 40 minutes. Remove from the fridge, lift out of the tin and cut into squares to serve.