Preheat oven 160°C fan forced. Grease a 14cm x 24cm (base) loaf pan. Line base and sides with baking paper, allowing 3cm overhang on all sides.
Using an electric mixer, beat butter, peanut butter and sugar until pale and creamy. Add egg, beat until combined. Transfer to a large bowl. Stir in banana.
Add flour and milk, in two batches, stirring until just combined. Spoon into prepared pan. Smooth the top. Bake for 1 hour, or until a skewer inserted into the centre comes out clean. Stand in pan 10 minutes.
Lift out onto a wire rack. Dust with icing sugar. Slice and serve warm or cold spread with butter or reduced fat cream cheese.
TIP: Two bananas should give you ¾ cup mashed banana)