Preheat oven to 350~. Grease a 9x13x2 inch pan. Combine flour and sugar in bowl. Melt butter, add cocoa and Pepsi. Pour over flour and sugar mixture. Stir well. Add buttermilk, eggs, baking soda and vanilla-mix well. Stir in marshmallows and pour into pan. Bake 40 minutes. Frost while warm.