Chocolate Sandwich Cookies with Malted Milk Chocolate Buttercream
1 1/2 cups all-purpose flour
1/3 cup dark or Dutch-processed cocoa powder
1/2 teaspoon instant espresso powder
1/2 teaspoon salt
1/2 teaspoon baking powder
1/4 teaspoon baking soda
1/2 cup (1 stick) unsalted butter, room temperature
1 1/4 cup granulated sugar
1 teaspoon vanilla extract
3 1/2 ounces (1 bar) milk chocolate, chopped
1/3 cup unsalted butter, room temperature
2 cups powdered sugar, sifted, plus more as needed
2 tablespoons heavy cream, plus more as needed
1 tablespoon malted milk powder
1/2 teaspoon vanilla extract
Preheat oven to 375ºF.
In a large bowl, sift together flour, cocoa, espresso, salt, baking powder, and baking soda. Set aside.
In a large mixing bowl or the bowl of a stand mixer, cream together butter and sugar until light and fluffy, 3 to 5 minutes. Add egg and vanilla and beat until smooth. Add dry ingredients and mix until no traces of the dry ingredients remain, scraping the sides of the bowl to ensure that everything is fully incorporated.
Drop dough by the tablespoon onto parchment-lined baking sheets. Roll into smooth 1-inch balls, then flatten slightly with the palm of your hand, forming even 1/2-inch-thick disks. Bake for 8 to 9 minutes or until cookies are puffed and set. Let cool for 5 minutes on the baking sheet (they will deflate slightly), then transfer to a wire rack to cool completely.
To prepare filling, melt milk chocolate in a microwave or double boiler over low heat until smooth. Remove from heat and let cool to room temperature.
In a large mixing bowl, beat butter until fluffy, 1 to 2 minutes. Add powdered sugar, 1/2 cup at a time, mixing well after each addition. Add cooled milk chocolate and beat until smooth.
Combine heavy cream and malted milk powder in a small bowl and mix until powder is dissolved. Add to milk chocolate mixture, along with vanilla and salt, and beat until light and fluffy, another 2 to 3 minutes. At this point you may need to add more cream or more powdered sugar as needed to achieve the desired consistency.
To assemble, spread or pipe about 1 tablespoon of filling onto half of the cookies, then top with remaining cookies. Press lightly until filling spreads to edges. Store cookies in an airtight container in the refrigerator for up to 5 days.