Preheat the oven to 180C (160C fan) and line a loaf tin with parchment paper.
Beat the butter and sugar together until light and fluffy.
Add the eggs one at a time, mixing well after each addition.
Stir in the mashed bananas until combined.
Fold in the flour and baking powder, then gently mix in the milk.
Pour half the batter into the prepared tin, spoon half the dulce de leche over it, and swirl with a knife.
Add the remaining batter, then swirl the rest of the dulce de leche on top.
Bake for 1 hour or until a skewer inserted comes out clean.
Cool in the tin for 10 minutes before transferring to a wire rack.