I have added this note afterwards. Having eaten a whole cake last night I found them far too sweet. If I made them again I would reduce the amount of sugar that went into the cake. The topping is nicely sweet but the cake was too much. Maybe half the amount of caster sugar as 140g does seem quite excessive.
For the chocolate frosting;
300 g icing sugar, sifted
100 g unsalted butter, at room temperature
40 g cocoa powder, sifted
40 ml whole milk
You will need a 12-hole cupcake/yorkshire pudding tray, lined with paper cases.
Preheat the oven to 170°C. Put the flour, sugar, baking powder, cinnamon, ginger, salt and butter in a freestanding electric mixer with a paddle attachment (or use a handheld electric whisk) and beat on slow speed until you get a sandy consistency and everything is combined.
Slowly pour the milk into the flour mixture, beating well until all the ingredients are well mixed.
Add the eggs and beat well (scrape any unmixed ingredients from the side of the bowl with a rubber spatula). Stir in the mashed banana by hand until evenly dispersed.
Spoon the mixture into the paper cases until two-thirds full and bake in the preheated oven for about 20 minutes, or until light golden and the sponge bounces back when touched.
Leave the cupcakes to cool slightly in the tray before turning out onto a wire cooling rack to cool completely.
To make the chocolate frosting, beat the icing sugar, butter and cocoa powder together in a freestanding electric mixer with a paddle attachment (or use a handheld electric whisk) on medium-slow speed until the mixture comes together and is well mixed.
Turn the mixer down to slow speed. Add the milk to the butter mixture a couple of tablespoons at a time. Once all the milk has been incorporated, turn the mixer up to high speed.
Continue beating until the frosting is light and fluffy, about 5 minutes. The longer the frosting is beaten, the fluffier and lighter it becomes.
When the cupcakes are cold, spoon the chocolate frosting on top and finish with the chocolate shavings.