Grease a 20 by 30 rectangular or 27 round cake pan with a mixture of flour and oil.
Preheat the oven to 180 degrees Celsius. If you have an electric oven, set it to 160 degrees.
Combine flour with baking powder and sift in the mixture 3 times.
Mix the filling ingredients, including chopped walnuts, cinnamon powder, and cardamom powder, and set aside. (Optional)
In a mixing bowl, beat the butter and sugar together with a mixer until light and fluffy.
Lower the mixer's speed, then add the oil gradually.
Add eggs, one at a time, beating 10 seconds for each egg. Add each new egg when the previous one has been absorbed.
Add the vanilla and keep beating for another 3 to 4 minutes.
Add the milk. Once you’ve stirred in the milk, stop the mixer
Add flour and baking powder in two to three steps. Stir with a whisk or silicone spatula.
If you don't want to use the filling mixture, pour all the batter into the pan.
If you want to add the filling to your cake, first pour half of the batter into the pan, then spread the mixture evenly across the bottom of the pan.
Then sprinkle the filling stuff including walnuts, cinnamon powder, and cardamom on the batter.
Using a piping bag, pour the remaining batter over the walnuts. Then smooth the batter with a spoon or silicone spatula.Do not flatten the batter too much, or walnuts may come out. Place the pan in the preheated oven for 30 to 40 minutes.
Do a toothpick test after 30 minutes. Insert a toothpick into the center of the cake; if it comes out clean, the cake