Preheat oven to 350 degrees. Grease and flour a 9 x 13 x 2" pan. Combine flour and sugar in a large bowl. Melt butter. Add cocoa and Pepsi Cola. Pour over flour mixture. Stir until well blended. Add buttermilk, beaten eggs, soda and vanilla. Stir in marshmallows. Mix well. Pour into prepared pan. Bake 40 minutes. Remove from oven and frost while still warm with Peanut Butter Frosting. Makes 20 2 x 3" servings.