Melt butter and sugar in a non-stick pan over low heat until dissolved.
Stir in condensed milk and bring to a boil until thickened into caramel.
Preheat the oven to 200°C.
Whisk butter and sugar together until light and fluffy.
Mix in cornflour and plain flour to form a smooth dough ball.
Line a 20cm x 30cm baking tin with parchment paper.
Flatten the dough and press it evenly into the bottom of the tin.
Bake the shortbread for 25 minutes.
Spread caramel over the shortbread and bake for another 6 minutes.
Melt chocolate in a bowl over a pan of simmering water.
Arrange sliced bananas on top of the warm caramel.
Pour melted chocolate over bananas and set aside to cool until firm.
Freeze chopped bananas on a baking sheet for one hour until firm.
Blend frozen bananas, vanilla, sugar, and buttermilk into a soft purée.
Cut shortbread into squares and serve topped with banana ice cream.