1.Heat the oven to 180C/fan 160C/gas 4. Butter and line the bases of 2 x 450g loaf tins with baking parchment. Melt the chocolate either in the microwave or in a small glass bowl set over a pan of barely simmering water. Stir until smooth and remove from the heat.
2.Cream together the softened butter and caster sugar until pale and fluffy. Gradually add the egg, beating well between each addition. Sift together the flour, baking powder and cocoa and fold in using a large metal spoon. Add the mashed banana and chocolate and mix well.
3.Divide the mixture between the 2 tins, Bake for about 45 minutes or until a skewer inserted in the middle comes out clean.