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.