Preheat oven to 325. Butter an 8" square baking dish. Line bottom with parchment paper, allowing 2" to hang over 2 sides. Butter parchment; set aside. Melt butter and chocolate together in a medium saucepan over medium-low heat, stirring until smooth. Remove from heat and stir in remaining ingredients.
Pour batter into prepared dish. Smooth top with a rubber spatula. Bake until a cake tester inserted into center comes out with moist crumbs, 30-35 minutes Let cool in pan ona wire rack 15 minute Lift out and let cool completley on rack. Cut into 16 2" squares.