FOR THE COOKIES
1 3/4 cups all-purpose flour
3/4 cup unsweetened cocoa powder (not Dutch-process)
1 1/2 teaspoons baking soda
1/2 teaspoon salt
1 tablespoon unsalted butter, softened
1/4 cup vegetable shortening
1/2 cup granulated sugar
1/2 cup packed dark-brown sugar
1 large egg
1 cup whole milk
1 teaspoon vanilla extract
2 ounces bittersweet chocolate, finely chopped
FOR THE FILLING
1 cup (2 sticks) unsalted butter, room temperature
2 cups sifted confectioner’s (powdered) sugar
1 jar marshmallow Fluff (also called Marshmallow Creme)
2 tsp vanilla extract
DIRECTIONS:
Prepare cookies: Preheat oven to 375 degrees. Sift together flour, cocoa powder, baking soda, and salt into a medium bowl; set aside.
Place butter, shortening, and sugars into the bowl of an electric mixer fitted with the paddle attachment. Mix on high speed until smooth, about 3 minutes. Add egg; mix until pale and fluffy, about 2 minutes. Mix in half the flour mixture, then the milk and vanilla. Mix in remaining flour mixture.
Drop about 2 teaspoons dough (I used a small cookie scoop) onto baking sheets lined with parchment paper, spacing 2 inches apart. Bake until cookies spring back when lightly touched, 12 to 14 minutes. Transfer baking sheets to wire racks and let cool 10 minutes. Remove cookies from baking sheets and transfer to wire racks using a spatula; let cool completely.
Prepare filling: In the bowl of a mixer fitted with the paddle attachment, cream together butter and sugar until pale and fluffy, about 3 minutes. Add Fluff and vanilla and continue mixing until well combined.
Pipe or spoon about 2 teaspoons filling on the flat sides of half the cookies. Top each with one of the remaining cookies, flat side down, and gently press together.
Melt chocolate in a saucepan over low heat, stirring until smooth. Transfer to a pastry bag fitted with a small round tip, or a small parchment cone, or a small sandwich baggie with a corner tip cut out. Pipe chocolate in a spiral pattern on top of each pie. Let chocolate set before serving, about 1 hour.