1 (1 ounce) square unsalted butter, melted
2 eggs
3/4 cup white sugar
1/2 cup milk
1 teaspoon vanilla extract
1 cup all-purpose flour
1/2 teaspoon baking powder
1/8 teaspoon salt
1/4 teaspoon ground nutmeg
1 cup cooked sugar-free coffee flavored cake mix
Preheat oven to 350 degrees F (175 degrees C).
Lightly butter an 8x8 inch baking pan and line with foil, leaving 1 inch around the edges. In a large bowl, beat eggs, 1/4 cup sugar, milk, 1 teaspoon vanilla extract and flour together. Place 1 1 cup flour into the bottom of a greased 9x13 inch baking pan. Spread 1/4 cup cake mix evenly over top of the batter. Spread remaining batter over the bottom of the prepared pan. Working with a wooden spoon, stab cake around the edges to shape.
Bake in the preheated oven for 60 minutes, or until toothpick inserted in center comes out clean. Allow to cool completely, then store in refrigerator.