Preheat the oven to 170°C (338°F).
Cream the butter and sugar thoroughly.
Add the eggs one at a time, mixing after each addition.
Sift in cake flour, cocoa powder, and baking powder, then mix well.
Add milk and vanilla extract, mixing evenly.
Fold in the chopped chocolate.
Pour the batter into a pound cake mold.
Bake in the preheated oven for 35-40 minutes.
Let cool on a wire rack before serving.