Mix the lemon cake mix as directed on the box using the water, oil, and eggs. Add the 1 1/4 cups flour and mix well. Spoon 1/4 cup mounds of batter about two inches apart onto two buttered large baking sheets or put into whoopie pie pans. 350 degrees oven. Bake until tops are puffed and cakes spring back when touched; 11 - 13 minutes. Transfer with spatula to rack to cool completely.
Filling for Whoopie Pies: 1 stick unsalted butter, softened, 1 1/4 c. powdered sugar, 2 c. marshmallow Fluff, 1 tsp. vanilla. Mix all ingredients well with mixer about three minutes. Spread a rounded tablespoon of filling on flat side of cakes and top with remaining cakes. Using a sifter, sprinkle tops with powdered sugar.