Crumble the Oreo® Chocolate Sandwich Cookies (4).
Slice the Banana (2) to 1 centimeter thick.
Sift together All-Purpose Flour (100 gram), Powdered Confectioners Sugar (1 tablespoon), Salt (to taste), and Unsweetened Cocoa Powder (1 tablespoon).
Use a cutter and finely chop the Unsalted Butter (3 1/2 tablespoon) with the dry ingredients until it becomes fine crumble.
Add in the Egg (2) and Water (1 tablespoon) and mix together until it forms dough. Wrap it with cling wrap and rest in fridge for at least 30 minutes.
Sift the Corn Flour (1 tablespoon) and Unsweetened Cocoa Powder (1 tablespoon) and mix together with Granulated Sugar (50 gram). Mix in the Egg (3) until combined.
Bring the Milk (200 milliliter) into shimmering. Mix 1-2 tablespoon hot milk with the mixture and eventually add in all milk and mix together. Bring the mixture back to the pot and keep stirring until thicken.
Quickly add in the Unsalted Butter (3 1/2 tablespoon) and keep stirring until combined. Put in fridge and set aside for use later.
Preheat oven to 190 degrees C (375 degrees F). Roll the chocolate pastry out about 3 cm wider than the tart mould. Cut the excess pastry out.
Use a fork to poke some hole on the pastry to let the steam out. Place a baking paper on top and fill with baking stone, bean or rice to prevent tart from rising.
Bake for 10 minutes, remove the baking stone or bean.
Brush a thin layer of egg on top of the pastry. Bake for another 15 minutes. Remove from the tin and cool down on the rack.
Make Chocolate ganache: use a double boiler to heat up the Thickened Cream (100 milliliter) and Milk (20 milliliter). When the milk become warm to touch then add in Dark Chocolate (70 gram).
Keep stirring until all chocolate is combined and no lump in the ganache. Stir in the Unsalted Butter (2 tablespoon) until combined. Cool down in the fridge.
Pour the chocolate ganache into the tart shell and fill about half of the tart. Arrange the banana in the tart.
Cover the top with Chocolate Custard. Sprinkle Oreo® Cookie crumble on top to cover.
Pipe the remaining chocolate custard on top, and dust some cocoa powder for finishing. Serve and enjoy!