Halloween Whoopie Pies

1)Preheat oven to 375 degrees. In a medium bowl, whisk flour, cocoa, baking soda, baking powder, and salt. 2)In a large bowl, using an electric mixer, beat 1/2 cup butter and brown sugar until light & fluffy. Add egg; beat until smooth. With mixer on low, alternately add flour mixture and applesauce, beginning and ending with flour mixture. Mix just until smooth (don’t overmix). 3)Drop dough by heaping tablespoons, 2 inches apart, onto two baking sheets. Bake until a toothpick inserted in the center of a cookie comes out clean, 10 to 14 minutes. Immediately transfer cookies to a wire rack to cool. 4)Make filling: In a large bowl, using an electric mixer, beat vanilla and remaining 1/2 cup butter until light & fluffy. Gradually add confectioners sugar, beat until smooth. 5)Spread bottom of half the cookies with 1 tablespoon filling each; sandwich with remaining cookies, pressing gently so filling oozes out slightly. Sprinkle exposed filling with nonpareils. Let rest 15 minutes to set.