Line the base of a deep 20cm/8in spring form tin with baking paper.
To make the base, crush the biscuits to fine crumbs and melt the butter over a low heat. Pour into a bowl and mix to combine. Spoon into the base of the tin and press down with the back of a spoon until level. Place in the fridge for 15 minutes.
To make the filling, heat the butter and sugar in a small non-stick saucepan and stir over a low heat until combined. Add the condensed milk and bring to the boil, stirring for 2–3 minutes – do not over-boil or it will become grainy and fudge-like. Add the vanilla and pour into the tin. Place in the fridge for at least 1 hour.
Whip the cream in a bowl until soft peaks form.
Arrange the banana slices in neat rounds on top of the toffee. Spoon or pipe the cream on top and level. Set in the fridge for an hour.
Remove from the tin, and zig zag the melted chocolate over the top or dust with cocoa powder.
You could use 1 x 397g tin of dulce de leche caramel instead of the toffee filling and skip step 3. Add a teaspoon of sea salt to make a salted caramel version. This recipe will also make 8 x 7cm/2in rings to make individual pies. The base and toffee can be prepared up to a day ahead and left in the fridge. Assemble with the banana and cream ideally on the day. Not suitable for freezing.