Preheat oven to 350 degrees F (175 degrees C). Grease and flour an 8 inch square baking pan.
Over low heat, melt chocolate and butter in a medium saucepan. Stir until smooth. Remove from heat and cool slightly.
In a large bowl, beat sugars and eggs until light and fluffy (about 2 minutes with electric mixer). Add melted chocolate, 3 tablespoons of the Grand Marnier, vanilla and the orange peel, mixing until well blended. Mix in the flour, salt and chocolate chips (on low speed if using an electric mixer).
Pour batter into prepared pan. Smooth the top. Bake for 30-35 until sides look done and top feels firm. Inserting a toothpick 2 inches from center should come out clean; however, center will jiggle and look undercooked.
Remove from oven and brush top with the remaining 1 tablespoon Grand Marnier. Cover pan with foil or plastic and cool completely. Cut into 2 inch squares. Store covered at room temperature or they may be frozen. Defrost covered at room temperature.