Preheat the oven to 180°C (356°F).
Sift together flour, cocoa powder, and baking powder.
Cream the butter and sugar together until light and fluffy.
Add eggs one at a time, mixing well after each addition.
Pour in milk and vanilla extract, and mix to combine.
Add the dry ingredients and mix until just combined.
Pour the batter into the pan and smooth the top.
Bake in the oven for 30–35 minutes.
Let it cool completely, then cut into desired shapes.