Preheat oven to 175 degrees celsius (fan-forced). Grease and line a 20cm X 20cm square cake tin with baking paper and set aside.
Place the butter (chopped) into the Thermomix bowl. Melt on Speed 2, 80 degrees, 3 minutes (or until melted).
Add the brown sugar, vanilla extract, mashed bananas and egg and mix on Speed 4, 10 seconds. Scrape down the sides of the bowl.
Add the plain flour and mix on Speed 4, 5 seconds. Scrape down the sides of the bowl and mix for a further 5 seconds.
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, place the butter, brown sugar and milk into the Thermomix bowl. Melt on Speed 2, 60 degrees, 3 minutes (or until slightly thickened. Allow to cool slightly.
Sift the icing sugar mixture into the bowl and mix on Speed 5, 5 seconds. Scrape down the sides of the bowl and repeat for a further 5 seconds.
Spread the caramel frosting over the top of the blondies and allow to cool completely.