In a small bowl, combine flour, baking powder and salt. Set aside. In another small bowl, beat eggs until foamy. Set aside. Melt the butter and chocolate in a large microwave safe bowl in 30 second increments until melted. Stir in the sugar, vanilla, cocoa powder. Carefully whisk in the food coloring (to desired color) until combined. Sir in the vinegar. Slowly stir in the eggs, then fold in the flour mixture until it is all combined. Do not overmix. Pour batter into prepared pan and spread evenly. Bake for about 30-35 minutes, or until a toothpick inserted in the middle comes out clean. Place on wire rack to cool completely.