Preheat the oven to 170°C (338°F).
Sift the cake flour, baking powder, and cocoa powder together.
In a bowl, cream the butter and sugar together.
Add the eggs one at a time, mixing well after each addition.
Add the sifted dry ingredients and espresso, and mix until just combined.
Fold in the chopped chocolate.
Pour the batter into a pound cake pan lined with parchment paper.
Bake at 170°C (338°F) for 40–45 minutes.
Cool and remove from the pan.