Preheat the oven to 180C/160C Fan/Gas 4. Grease and line a 23x30cm/9x12in traybake tin with baking paper.
For the sponge, measure the cocoa powder and boiling water into a large bowl and mix until smooth. Mix the butter and sugar together until combined. Then add each egg one at a time and mix until all four are incorporated.
Then sieve in the flour and baking powder then mix again until combined. Pour the mixture into the line tin and level out.
Bake in the middle of the oven for about 25 minutes, or until the cake is coming away from the sides of the tin and is springy to the touch in the middle. Leave to cool in the tin on a wire rack.
Once the cake is fully cooled, remove from the tin and turn on to a work surface. Using a pumpkin cookie cutter, cut out pumpkins from the sponge.