This recipe will make your arm hurt. But it is soooooo good. Really worth it vs. brownie mix (though I make those often enough too).
2 1/2 sticks unsalted butter
2 1/2 cups sugar
1 3/4 cup unsweetened cocoa powder (I used natural, dutch would work too)
1/2 teaspoon salt
1 teaspoon vanilla
4 cold large eggs
1 cup all-purpose flour
Grease the bottom of a 13 x 9 pan. Preheat oven to 325°F.
Combine the butter, sugar, cocoa, and salt in a medium heatproof bowl and set the bowl in a wide skillet of just simmering water (I actually do this right in my dutch oven and haven't ruined a batch yet) . Stir frequently, and continue until the mixture is smooth and hot enough that you want to remove your finger fairly quickly after dipping it in to test. You will actually see the cocoa powder start to get smooth and shiny.
Remove the bowl from the skillet and set aside briefly until the mixture is only warm, not hot.
Stir in the vanilla with a wooden spoon. Add the eggs one at a time, stirring and stirring adn stirring after each one.
When the batter looks thick, shiny, and well blended, add the flour and stir until you cannot see it any longer, and THEN beat vigorously for another 40 strokes with the wooden spoon.
Spread the mixture in the prepared pan.
Bake until a toothpick plunged into the center emerges slightly moist with batter, 25 - 30 minutes. Let cool completely on a rack.
It's fairly easy to make a gluten free version of this - because there is so little flour to start, just replace it with a gluten free all purpose from the grocery store. If you half the recipe, bake in a 8 x 8 pan, and bake for 20-25 minutes.