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 for 3 minutes, Speed 2, 80 degrees (or until melted).
Add the brown sugar, vanilla extract, mashed banana and egg and mix for 10 seconds, Speed 4. Scrape down the sides of the bowl.
Add the plain flour and mix for 5 seconds on Speed 4. 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 for 3 minutes on Speed 2, 60 degrees (or until slightly thickened. Allow to cool slightly.
Sift the icing sugar mixture into the bowl and mix for 5 seconds on Speed 5. Scrape down the sides of the bowl and repeat for a further 5 seconds. If the frosting is too thick to spread, add an extra 1-2 tablespoons of milk and continue mixing.
Spread the caramel frosting over the top of the blondies and allow to cool completely.