Preheat the oven to 350º. Generously butter a 9 x 13 inch baking dish or pan. Or use two 8 x 8 inch pans, or two 9 inch round cake pans.

In a large bowl, melt the butter - either on low power in the nicrowave, or over a pot of simmering water.

Mix in the cocoa powder until fully incorporated and allow the mixture to sit for 10 minutes.

Then, one a time, mix in the sugar, each of the eggs and the vanilla.

In a smaller bowl, whisk together the flour, baking powder and salt.

Add the dry ingredients to the butter mixture and fold together, just until combined.

Spread the batter evenly in the prepared dish.

Bake for about 25 minutes, until a toothpick inserted near the center comes out with just a few moist crumbs, but no batter at all.