Preheat oven to 350 degrees. Butter an 8x8" baking dish.
Combine flour, and salt in a bowl, and mix well.
Melt the butter and chocolate in a double boiler (I just use a metal pan set in a saucepan full of water.) When the chocolate is nearly melted, stir until smooth, remove from the heat, and set aside.
Beat the sugar and eggs until extremely creamy, about 5 minutes with a hand mixer. Then add vanilla extract and chocolate and mix.
Blend in flour. Spread the mixture in the baking dish as evenly as possible.
Bake in the center of the oven until a toothpick comes out nearly clean, about 30 minutes. Let cool for 20-30 minutes before cutting.
Chef John says you can double the batch size and bake in a 9x13" pan. They'll be a little thicker.