Cake without baking powder made 2 layers in a cake. Why?

Yesterday I baked a chocolate cake. Fifteen minutes after it went in the oven I realised I'd left out the baking powder, so I quickly made another one, this one with baking powder.

This cake recipe only uses 2 eggs (unlike most cakes I've baked that use 4 ) and only 2 teaspoons of baking powder instead of a whole packet (3-4 teaspoons). This means it will grow less (eggs grow when cooked, as well as baking powder).

This means there is more batter and it is more liquid, instead of the typical thick batter that grows a lot in the oven. In those cakes in the past (thick batter, 4 eggs and a whole packet of baking powder), whenever I forgot to put baking powder, it literally ended up in a hard and one-finger thin cake, difficult to eat.

But this cake baked quite well. After it was baked I made another one, exactly the same but with baking powder. The difference it was basically that it was a bit (just a little) bigger and quite spongier. But the MOST representative thing was that it all looked homogeneous (like a cake should, I did not take a picture of the second one). But look at the no-baking-powder cake. You can literally see 2 different layers of cake (one more choco-flavourful). This did not happen to the second one.

Does the baking powder have something to do with this? I guess so, but I do not fully understand how can baking powder solve the issue of the "mix batter". Like, my logic tells me once you put the mold in the oven, if it is not homogeneous, the thicker part will go down, but it did not happen in the second one...

Ingredients: Eggs (2), cocoa powder, sugar, flour, milk, water (sounds weird but gives the thing!), vegetable oil, some melted butter!