Preheat your oven to 180 degrees Celsius (350 degrees Fahrenheit). Line a loaf pan with baking paper and set aside.

In a microwave safe bowl, combine the butter and peanut butter, and microwave to melt completely. Stir to incorporate.

In a large bowl, combine the mashed banana and peanut butter mixture with a wooden spoon. Add the caster and brown sugar, beaten egg and vanilla to this, stirring to incorporate all the ingredients.

Add the flour, bicarb of soda and salt to the mixture, and stir until just combine. Be careful not to overbeat the batter.

Pour into the prepared loaf pan, ensuring that it is evenly spread. Bake for 1 hour, or until a cake skewer inserted into the middle comes out clean. Allow to cool slightly, remove from the pan, and cut into even slices. Best served still warm.