In a food processor, blitz the ripe bananas until smooth. You can also use the back of a fork.
In a stand mixer, beat together the butter and sugar until light and fluffy. To this add the eggs one at a time, beating between each addition. Next add the vanilla, buttermilk and bananas. In a separate bowl mix together the flour, baking powder and a pinch of salt. Add this flour mix to the wet mix and combine, making sure not to overmix.
Pour into the prepared tins and bake in the oven for 40 minutes until golden brown and cooked through. Allow to cool for 10 minutes before removing from the tins andcooling on a wire rack.
Make the banana pudding filling: Using an electric whisk beat together the banana pudding mix and cream cheese until smooth, and then beat in the double cream until the mixture has loosened slightly. Set to one side.
Make the dulce de leche custard: Heat the milk until scalding point. In a bowl mix together the egg yolks, vanilla, sugar and plain flour. Slowly pour over the hot milk whisking continuously until all incorporated. Back in the pan, add the caramel and gradually pour in the hot milk, whisking continuously, until the custard has thickened significantly. Then remove the mixture from the heat and pass it through a sieve into a bowl to cool.
In a stand mixer beat the butter until light and fluffy. Then add the cooled custard a tablespoon at a time and beat again until incorporated. Set aside.
Assemble your cake: Take your cooled cakes and if necessary, level off so they are straight. Place the first one down on the turntable and on top smooth on a layer of the banana pudding filling with a spatula. Then slice a banana and add a layer of banana on top. Place the second sponge cake on top of this and repeat. With the final layer, Use the dulce de leche custard cream to ice the top and sides of the whole cake. Decorate with the crushed Biscoff biscuits and the remaining dulce de Leche from the can and serve.