Preheat the oven to 180°C (356°F).
Sift together the flour, cocoa powder, baking powder, and salt.
Whip the butter and sugar until creamy.
Add eggs one at a time and mix well after each addition.
Alternately add the sifted dry ingredients and milk, mixing between each addition.
Pour the batter into the cake pan.
Bake in the preheated oven for 30 to 35 minutes.
Let cool, then cut into 8 slices.