In another bowl, cream the butter, the sugar, and cocoa powder in an electric mixer. Add the egg and vanilla and beat until smooth. At low speed or with a wooden spoon, stir in the dry ingredients. Form a disc with the dough. Cover with plastic wrap. Refrigerate for 30 minutes.
On a floured work surface, roll out the pastry and line a 23-cm (9-inch) pie plate with removable bottom.
With the rack in the lowest position, preheat the oven to 200 °C (400 °F). Bake for about 15 minutes or until the crust is thoroughly cooked.
In a small saucepan, bring the cream, mashed bananas, and brown sugar to a boil.
In a food processor, pour the hot cream over the chocolate. Pulse for a few seconds at a time, until the chocolate has melted. Add the egg and pulse until the mixture is smooth. Spread the ganache into the crust. Refrigerate for 3 hours or until the filling is firm.
In a bowl, beat all the ingredients with an electric mixer until stiff peaks form.
Cover the pie with the whipped cream and top with chocolate shavings. Refrigerate until ready to serve.