To make the crust: Place butter, cocoa powder, and salt in a medium bowl and melt in the microwave. Mix well. Add crumbs, and mix until combined and moistened. Using your fingers, press the mixture into the bottom of a 9-inch tart pan (I use a pan with removable bottom) and up the sides to form an even layer. For a crispy crust, bake the crust at 350ºF/180ºC for 8 minutes. Allow to cool before adding the filling. If you don’t want to bake the crust, cover the crust with plastic wrap and place in the fridge to chill for one hour, until firm, before adding the filling.
To make the filling: In a medium heatproof bowl, place chopped milk chocolate, Nutella, and heavy cream. Microwave in 30-second intervals until melted and smooth. Pour mixture over bananas and refrigerate until set, at least 4 hours or overnight. You can make the tart a day ahead of time.