1. In a small cup, combine milk and white vinegar; set aside
2. In a small bowl, combine flour, sugar, cocoa powder, baking powder, and salt.
3. In a separate small bowl combine the vegetable oil, egg, vanilla, and food coloring. Add the oil mixture and the milk mixture to the flour mixture. Stir until well-combined. Divide batter into two mugs, being sure to keep from filling either past the half-way mark.
4. Microwave each mug individually for 1 1/2 minutes. When done, the cake should be firm and springy, not doughy. If it isn't done, microwave in 15-second intervals up to 2 1/2 minutes total, checking after each 15-second interval. Allow to cool for 3 minutes while you make a heart stencil.
5. To make the stencil: Cut a circle of parchement paper that will fit inside the rim of the mug and cover the surface of the cake. Cut a heart out of the center of the circle. Place stencil on the cake and shake on a powdered sugar heart.