Melt the chocolate in a double boiler or in a bowl set over a pan of simmering water, taking care not to let the water touch the bowl. Set aside.
Cream the butter and sugar until fluffy and pale. Gradually add the eggs, one at a time, beating well between each addition.
Sift the flour, baking powder, and cocoa powder and fold into the wet ingredients (I like to use a flattish metal spoon for this).
Add mashed bananas and melted chocolate and fold in well.
Place in a greased and lined loaf pan and bake in a pre-heated 180C oven for 50-60 minutes or until a wooden toothpick inserted in the middle comes out clean. Cool on a rack, then wrap in baking paper or foil.