Heat oven to 350 degrees F. Coat an 8" x 8" square baking pan with cooking spray.
In a large saucepan, melt the butter over low heat. Remove pan from heat. Stir in the cocoa powder with a wooden spoon until smooth. Beat in the sugar, eggs and vanilla. Add the flour and salt and stir until flour is fully incorporated.
Spread batter evenly into prepared baking pan. Bake 25 to 30 minutes. Do not over-bake!