1/2 cup butter, softened
3/4 cup white sugar
1 cup packed brown sugar
1 teaspoon vanilla extract
4 egg whites
1/4 teaspoon cream of tartar
2 drops orange liqueur
Preheat oven to 350 degrees F (175 degrees C). Grease and flour an 8x8 inch dish. In a large bowl, stir sugar, milk, brown sugar and vanilla extract together until smooth. Stir in egg whites until the whites form stiff peaks. Press fudge into the bottom of a 9x13 inch baking dish. Spoon orange liqueur evenly over the top, then top with fudge.
Bake for 25 to 30 minutes in the preheated oven, or until a knife inserted into the cake comes out clean. Cool completely before cutting into squares each of 3/4 inch deep. Leftover cake can be stored in a cool dry place (covered) for a week or longer.