Take a bowl and add oil, sugar and buttermilk in it.
Mix it well until the sugar dissolves completely.
Now add vanilla essence and mix it again.
Add mashed bananas and mix properly.
Now take a big sieve and place it over the bowl.
Add whole wheat flour, baking powder, baking soda and salt in it.
Sieve these dry ingredients directly into the wet ingredients already placed in the bowl.
Mix everything properly leaving behind no lumps. But don't overdo.
Spread this entire mixture in the cake tin over the layer of round banana pieces.
Gently tap the cake tin so that excess air bubbles are released and surface of the batter becomes plain.
Transfer the cake tin in oven and bake in the middle rack of Oven for 50 to 55 minutes at 180°C with both the rods on. You can also bake it in microwave on convention mode.
Check the done-ness by inserting a toothpick or knife in the cake and if the toothpick/knife comes out clean, it means your cake is ready.
Your banana upside down cake is ready.