Line a 17-18 cm (7 inch) spring form pan with a circle of baking paper.
In a bowl combine cookie crumbs and ground walnuts and stir. Use a kitchen scale to divide them in two equal parts (if you don’t have scale, use a measuring cup).
Combine one half of the cookie crumb mixture with the melted butter and mix well. Press it to the bottom of the prepared spring form pan. Place the pan in the freezer while preparing the cheesecake filling.
In a large bowl with an electric mixer beat together mascarpone, vanilla and powdered sugar until smooth.
Melt the chocolate: preferably on a double boiler (over a pan of gently simmering water). Cool it down a bit.
Using a rubber spatula fold in the melted chocolate. Start by adding a bit of the mascarpone mixture to the chocolate (1 tablespoon at a time) then mixing energetically. repeat 2-3 times and then fold the chocolate mixture into the remaining mascarpone.
Spoon about ⅓ of the mascarpone mixture in the cake pan, over the crust. Arrange on top half of the banana slices and sprinkle them with about ⅓ of the remaining crumb mixture.
Spread another ⅓ of the mascarpone mixture and cover with a second layer of bananas and cookie crumbs. Spread on top the last part of the mascarpone and smooth it with a spatula.
Cover the cheesecake with cling film and let it stay in the fridge for at least 4 hours.
Before serving decorate with the remaining fresh bananas, crumbs and melted chocolate.
If you don't have much experience with chocolate, choose a kind with about 60% cocoa content as it will be easier to mix it with the mascarpone. For real chocolate lovers 80-90% cocoa content is great but you need to be extra careful to avoid the chocolate hardening while being mixed in the filling.