Preheat oven to 350°F. Prepare mini whoopie pie pan with vegetable pan spray.

In large bowl, stir together flour, baking powder and salt.

In large bowl, beat butter and sugar with electric mixer until light and fluffy. Add egg whites and vanilla extract and beat until well combined. Alternately add flour mixture and milk in three additions, beating until just combined. Spoon one tablespoon batter into each cavity.

Bake 9 to 11 minutes or until tops of cakes spring back when touched. Cool in pan 3 minutes. Cool completely on cooling grid.

In separate small bowls, tint 1 cup buttercream red, 1 cup blue and reserve 1 cup white.

To assemble: Place one cake in bottom of treat pop. Pipe a swirl of blue icing from back edge following the curve of the container to the front, then filling in the center; add sprinkles. Add second cake. Pipe a swirl of white icing; add sprinkles. Top with another cake. Pipe a swirl of red icing. Top with Patriotic Sprinkles or Rocket Treat Pops Toppers.