Preheat oven to 375°F (190°C). Spray bottom of 8x8 inch pyrex dish with cooking spray (or cover it lightly with oil or butter). Mix the dry ingredients and set aside. Cream the butter and sugar together. Mix in the eggs and vanilla. Quickly stir in the dry ingredients, and then spread in pan and put in oven.
Bake for about 30 minutes. The top should be starting to look dry and the edges should be starting to darken. Take out of oven and let cool before cutting into.
8 by 8 inches is roughly 20 cm by 20 cm. I use PAM cooking spray, I was a bit resistant to it until I read the label and discovered that it was basically just aerolized canola oil.
I initially had a dutched/natural cocoa powder mix, and used 1/2 teaspoon baking powder. I switched to naturally processed cocoa powder, and it wasn't quite right, so I substituted baking soda for the half the baking powder.