1- First of all, put your all ingredients on your work surface. For the cake: Mix eggs yolk with granulated sugar and vanilla extract until creamy and then set aside. Then take another bowl and put eggs' white and salt in it and mix them until they turn to white and frothy and set aside. Then take eggs yolk mixture and add baking powder, vegetable oil, cocoa powder, flour and mix them well. Lastly add eggs white and salt mixture into mixture and mix all of them well. Then stir until you get a smooth batter.
2- Then, place parchment paper into your cake-pan/30 cm (nearly 9" inch) or place parchment paper into your cake mold and lock it and pour you cake mixture into a cake-pan. Bake at 180°C (350°F) oven until a toothpick inserted in the center of cake and comes out clean (nearly 40-45 minutes). Then, remove from oven and let it cool completely before inverting on a platter.
3- For the custard: Stir the milk, egg, butter, granulated sugar, starch and vanilla in a pot. Then, cook mixture until it thickens by stirring constantly. Then, remove the pot and let it get warm.
4- For Chocolate Ganache: Chocolate Ganache is strictly chocolate that is melted and thinned out with milk cream. In a small pot, heat milk cream till bubbles form at the side. Do not allow it to boil. Then, pour hot milk cream over the bitter chocolate and milky chocolate and whisk till chocolate melts. Allow it to cool slightly and thicken before use (or place your chocolate ganache into fridge for cooling 2-3 minutes).
5- Then, flatten the top of the cake by using an knife (This separated part will also be used to cover the top of the cake). Then, cut the cake in half, so you can get two half pieces, and then sprinkle warm milk and a pinch of sugar mixture (or warm water and a pinch of sugar mixture) all over the each piece of cake for moistining it.
6- Then, spread some of the custard mixture on top of the first layer, then place the sliced pieces of bananas and colorful chocolate chips on it. Then place the second layer on top. Then, spread the rest of the custard mixture mixture on top and on the side of the cake. Then pour your chocolate ganache all over the cake.
7- Lastly for decoration of cake; place the rest of the sliced pieces of banana and colorful chocolate chips on topping. And then place your chocolate and banana layered cake into refrigerator for a while (15-20 minutes).
8- Then serve. A cup of hot tea or our Ottoman-palace-lemonade- goes well with this birthday cake.
Enjoy!