Combine flour and sugar in a 2 12-quart bowl set aside Combine butter cocoa and cola in a 2-cup glass measuring cup Microwave at High 2 to 2 12 minutes
Stir cola mixture into flour and sugar
Add buttermilk egg soda vanilla and marshmallows beat well
Pour into a greased 13 x 9-inch baking dish
Spread marshmallows evenly in batter
Microwave at High 10 to 12 minutes rotate dish 14 turn at 2-minute intervals