For crust, sift together 1 ½ cups of all-purpose flour, 3 tablespoons of cocoa powder, 2 tablespoons of powdered sugar, and ½ teaspoon of salt in a mixing bowl. Sprinkle ½ cup of cold unsalted butter and 2 tablespoons of vegetable shortening over the flour mixture. Use a pastry blender or your fingers to cut in the fats until the mixture resembles pea-sized crumbs. Make a well in the center of the dry mixture and sprinkle in 3 tablespoons of ice water. Combine until the dough starts to clump together. Wrap the dough in plastic wrap and chill.
Preheat your oven to 375 degrees F. Roll out the chilled dough and fit it into a 9-inch pie pan. Freeze for about 10 minutes. Line with parchment and fill with weights. Bake for 15 minutes, then remove the weights and poke the crust. Bake for another 10-15 minutes until dry and matte.
In a saucepan, whisk together ⅔ cup of granulated sugar, 3 tablespoons of flour, 2 tablespoons of cornstarch, and ¼ teaspoon of salt. Gradually whisk in 1 cup of milk and heat until steaming. In another bowl, whisk 4 egg yolks and temper with the hot mixture.
Return to heat and stir until thickened. Remove from heat and stir in 2 tablespoons of butter and 1 teaspoon of vanilla. Divide filling, adding ¼ cup of chopped dark chocolate to one half.
In the cooled pie crust, layer 2 sliced bananas, pour the plain filling over them, then the chocolate filling. Chill the pie for at least an hour.
Whip 1 ¼ cups of heavy cream with 3 tablespoons of sugar and ½ teaspoon of vanilla until peaks form. Spread over the pie and garnish with chocolate curls.