Preheat oven to 160°C (140°C fan-forced). Grease and line a loaf tin with non-stick baking paper.
Place the butter and sugar in an electric mixer and beat for 10 minutes or until pale and creamy. Gradually add the eggs and beat well. Fold through the flour, baking powder, bicarbonate of soda and mixed spice and stir well to combine. Stir in the banana and golden syrup.
Place half the mixture in a separate bowl, add the melted chocolate and mix well to combine. Place alternating spoonfuls of each mixture in the prepared loaf tin and swirl with a butter knife.
Bake for 60-70 minutes or until cooked when tested with a skewer. Allow to cool completely in the tin.