Heat the oven to 180C/160C fan/gas 4. Butter a 900g loaf tin and line the base and sides with baking parchment. Set aside.
Beat the butter and sugar with an electric whisk until light and fluffy, then slowly whisk in the beaten egg, followed by half the flour. Fold in the remaining flour, the baking powder, mixed spice, mashed bananas and mincemeat.
Pour into the tin and bake for 1 hr until a skewer inserted into the middle comes out clean (cover with foil if it darkens too quickly). Leave to cool in the tin for 10 mins, then transfer to a wire rack and leave to cool completely.
To make the whipped butter, beat the butter, honey and cinnamon using an electric whisk until pale and fluffy. If you like, toast slices of the bread and spread with the butter while warm. The bread will keep in an airtight container for three days, and the butter chilled for up to five.