Place the cans of condensed milk in a deep saucepan and cover with water, topping up with hot water as necessary. Bring to the boil, then reduce the heat and cover with a lid. Leave to simmer for two hours. Remove from the water and cool completely. When you open the cans you will find it has turned to a golden sticky caramel.
Preheat the oven to 170C/340F/Gas 3.
Cream the butter with the sugar in a large bowl until it is light and fluffy. Sift the cornflour and plain flour into the bowl. Mix and gently knead the dough until it comes together in a ball.
Line a 20cm/8in x 30cm/12in baking tin with non-stick baking parchment. Roll out two-thirds of the dough to fit the tin and lay it inside, pressing it neatly into the edges.
Spread three quarters of the condensed-milk caramel evenly over the base (the rest will keep in a covered bowl in the fridge for up to two weeks). Lay the banana slices over the caramel and then crumble the remaining third of the dough over the top.
Bake for 20 minutes. The caramel should have bubbled up a little among the dough and the top of the shortbread should be golden-brown. Leave it to cool in the tin for five minutes before cutting it into squares. Allow to cool completely before removing from the tin.
To serve, reheat the shortbread squares. Serve with banana ice cream and a drizzle of caramel sauce. Decorate with a sprig of fresh mint.