You can make this cake a day or two ahead as the bananas will keep it really moist.
Heat the oven to 180C/fan 160C/gas 4. Butter and line the bases of 2 x 450g loaf tins with baking parchment.
To make the streusel topping, rub the butter into the flour, then mix in the demerara sugar and chopped pecans. 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.
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.
Divide the mixture between the 2 tins and top each with the streusel. Bake for about 45 minutes or until a skewer inserted in the middle comes out clean. Cool on a rack, then wrap in baking parchment or foil.