Whisk sugar, buttermilk, canola oil, egg and vanilla in a large bowl until smooth. Whisk in flour, cocoa powder, baking soda and salt.
Butter 4 microwave-safe mugs and divide 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 insert marshmallow cream.