To make a batch, whisk 1/2 cup each sugar and buttermilk, 1/4 cup vegetable oil, 1 egg and 1/2 teaspoon vanilla in a large bowl until smooth. Whisk in 1⁄3 cup flour, 2 tablespoons unsweetened cocoa powder, 1/8 teaspoon baking soda and a pinch of salt. Butter 4 small microwave-safe mugs and divide the batter among them. Microwave, one at a time, until a toothpick comes out clean, about 2 minutes. Make a slit in the center of each cake and pipe marshmallow cream into the slit. Top the cake with warmed chocolate frosting, then pipe white frosting squiggles on top.