Preheat oven to 180 degrees C and butter a pie dish.
Make the pastry: place the flour in a bowl and form a well in the centre. Pour in the melted butter and stir to make small crumbs. Add the cocoa and mix well. Add a little water and mix by hand until it forms a cohesive mass. Shape into a ball, cover with a clean cloth and let rest in a cool place 30 minutes.
Roll out the pastry and line the pie dish. Prick with a fork, cover with foil and add dry rice or beans to prevent the dough from rising when cooking. Blind bake 20 minutes and let cool, removing the beans.
To make the filling, heat the single cream in a saucepan until simmering. Remove from heat and pour over the chocolate. Cover with a lid and wait 5 minutes, then mix with a wooden spoon until smooth. Place the bananas in the pie shell, pour over the chocolate and chill until firm.