1. Preheat oven to 350°. Line a 13- x 9-inch pan with aluminum foil, allowing 2 inches to extend over sides; lightly grease foil.
2. Bring 1 inch of water to a simmer in bottom of a double boiler. Place chocolate and butter in top of double boiler. Cook, stirring occasionally, 5 to 6 minutes or until melted. Cool 10 minutes; transfer to a large bowl. Stir in sugar until blended. Stir in flour and next 3 ingredients just until blended. Pour batter into pan.
3. Bake at 350° for 32 to 35 minutes or until set. Cool in pan 30 minutes. Freeze 2 hours; cut into squares or triangles. Press tops with gold leaf, if desired.
Make-Ahead Tip: Freeze up to 2 months. If using gold leaf, add it just before serving.