8 tablespoons (1 stick) unsalted butter
1 1/4 cups sugar
2 large eggs
1 teaspoon vanilla extract
1/2 cup Dutch-process unsweetened cocoa powder
1 cup all-purpose flour
1/2 teaspoon salt
1 teaspoon baking powder
1/2 teaspoon baking soda
Filling:
4 tablespoons (1/2 stick) unsalted butter
1 teaspoon vanilla extract
4 cups powdered sugar
heavy (whipping) cream
In a large bowl, with an electric mixer, cream the butter with the sugar until light. Beat in the eggs and vanilla. Sift the cocoa, flour, salt, baking powder and baking soda together. Stir the dry ingredients thoroughly into the butter mixture. Cover the bowl with plastic wrap and refrigerate at least 2 hours, preferably overnight.
Preheat the oven to 375 degrees. Butter two baking sheets or line them with silicone baking mats. Using a teaspoon measure, roll level teaspoons of the chilled dough into balls and place them 2 inches apart on the baking sheets. Bake, one sheet at a time, for 10 to 15 minutes, or until the cookies are puffed and the surfaces are slightly dry and cracked. Cool completely on racks.
In a bowl, with an electric mixer, cream the butter until light. Beat in the vanilla and powdered sugar. If the filling is not creamy, beat in heavy cream a bit at a time and continue to beat until the consistency is like creamy frosting. To assemble the cookies, carefully spread about 1/2 tablespoon filling all over the flat side of one cookie, then top with the flat side of another cookie. Serve immediately.
Source: Goldy’s Kitchen Cookbook