Preheat oven to 175 degrees celsius (fan-forced). Grease and line a 20cm X 20cm square cake tin with baking paper and set aside.
Beat the melted butter and brown sugar together until well combined.
Add the vanilla extract, mashed bananas and egg and beat until combined.
Add the plain flour and stir through until combined.
Pour the mixture into the prepared tin and bake for 25-30 minutes or until cooked through.
Allow to cool in the pan.
To make the caramel frosting, melt the butter in a microwave-safe bowl on medium power (alternatively, you can use a saucepan).
Add the brown sugar and milk and stir together. Place back into the microwave for 2-3 30 second bursts or until slightly thickened. Allow to cool slightly for a few minutes.
Sift the icing sugar mixture over the top of the mixture and mix together with a whisk.
Spread the caramel frosting over the top of the blondies and allow to cool completely.