Chocolate Sandwich Cookies with Malted Milk Chocolate Buttercream

INGREDIENTS

• Cookies:
• 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 egg
• 1 teaspoon vanilla extract
• Filling:
• 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
• pinch salt