Preheat your oven to 180°C (160°C fan)/Gas 4. Grease and line a 30x23cm traybake tin with baking parchment.
In a small bowl, mix the cocoa powder with the hot water to form a smooth paste. Allow it to cool slightly
In a large bowl, combine the cocoa mixture with the butter, sugar, self-raising flour, baking powder, eggs, and milk. Mix well until you have a smooth batter. You can use a hand mixer or a wooden spoon for this step.
Pour the batter into the prepared tin and level the surface with a spatula.
Bake in the preheated oven for 25-35 minutes, or until the cake is well-risen, springy to the touch, and pulling away slightly from the edges of the tin.
Leave the cake to cool in the tin for 10 minutes before transferring it to a wire rack. Remove the baking parchment and let the cake cool completely.
