1. Put the unopened cans of condensed milk in a deep saucepan and cover with cold water. Bring to the boil, the reduce the heat and put on the lid. Leave to simmer rapidly for 2 hours, topping up the water if necessary. Cool down completely before you open the cans, to find a golden sticky caramel. (Once cooked, a can of caramelised condensed milk will keep in the fridge for 2 weeks.)
2. Preheat the oven to 200 C / 400 F.
3. Cream the butter with the sugar until light and fluffy.
4. Sift together the flours, then combine with the butter and sugar to form a dough. Gently knead until it all comes together in a firm ball.
5. Line 20 x 30cm baking tray with non-stick baking parchment. Roll out two-thirds of the dough to fit the tin, and press it right into the edges of the tin.
6. Spread three-quarters of the caramelised condensed milk evenly over the base. Add the sliced bananas, then crumble the remaining third of the dough over the top.
7. Bake for 20-25 minutes until the top of the shortbread is golden. Some of the caramel will bubble up through the topping - this is good!
8. Leave to cool for 5 minutes in the tin, before cutting into small squares. Leave to cool completely in the tin.
9. Eat. Smile. Eat some more.