Preheat the oven to 175ºC. Grease and line a 2lb loaf tin.
In the mixing bowl of a stand mixer (or large bowl, if you don't have a stand mixer), cream together the stork/butter and sugar until light and fluffy.
Add the bananas. Crack the eggs in another bowl or jug and add them to the mix, one by one. Mix until well incorporated.
Add the flour and the mixed spice and mix until just incorporated - don't overmix.
Add the chocolate chips and put in the oven. Bake the cake for 35-40 minutes, checking after 30 minutes to make sure it's not too brown on top. If it starts to brown too much, pop some foil over the top of it to protect it. You'll know when the cake is cooked as a skewer will come out clean.
When the cake is cooked, remove from the oven and leave to cool in its tin for 5-10 minutes. Turn out from the tin and leave to cool completely on a wire rack before covering in buttercream.
To Make the Buttercream
In the bowl of your stand mixer (or a large bowl if you want to use a hand mixer) put the butter and icing sugar. Mix on a slow speed to start with (otherwise you'll end up with a cloud of icing sugar everywhere) then gradually increase the speed until it's all mixed together.
Add the vanilla extract and mix through.
Using a palette knife, apply the buttercream to the top of the cooled cake. If the cake isn't cool, the buttercream will melt into the cake or run off the sides! Sprinkle some chocolate chips over the top, and get some plates ready!