Preheat the oven to 200ºC/180ºC Fan/Gas Mark 6. Line and grease a 2lb loaf tin with baking paper.
Mix the coffee powder with 1 tbsp boiling water and set aside to cool.
In a large bowl, whisk the butter with the sugar. When light and fluffy, whisk in the egg. Add in the milk, cooled coffee, and vanilla extract and whisk again.
Gently fold in the mashed banana, flour and cinnamon.
Spoon the mixture into the tin, level the surface and bake for 40-45 minutes until cooked through. It's ready when a skewer passed into the centre of the cake comes out clean.
When cooked, remove the cake from the oven, leave to cool in the tin for 5 minutes, then turn out onto a rack and leave to cool completely.
If you want to decorate the cake, whip the cream with the coffee powder and vanilla extract. Spread over the cooled cake and top with banana slices just before serving.