Why do my cakes taste like cinnamon? I baked a cake using eggs, butter, milk, vanilla extract, flour, cocoa and baking soda. 
I baked it at 450F for 30 mins. I tested it and the knife came clean. It looked okay but tasted horrible, like it contained cinnamon.

Then I tried another recipe which had the same ingredients but I was told to add baking powder. This one was baked at 325F for 30 mins in the middle rack of the oven. 

However, when we ate it, it tasted horrible and like it had cinnamon, though I had put no cinnamon in either.

Why are my cakes coming out like this? How do I fix it?
From your list of ingredients, I can tell you are missing at least one ingredient. 

Sugar

Or any kind of sweetener. Cocao powder by itself is very bitter and acidic. Without the sugar it's not too surprising the cake taste bitter and slightly sour.