Preparation Steps

• In a stand mixer with a paddle attachment, cream the softened butter and 2 cups powdered sugar on medium speed until light and fluffy, about 2 minutes.
• Add the cocoa powder, salt, and vanilla extract. Mix until fully incorporated. Add 0.75 cup of flour and mix until combined.
• Gradually add the remaining flour, up to 1 cup, 0.25 cup at a time, until the dough forms a stiff, non-sticky mass that may crumble slightly—this ensures a crisp cookie.
• Divide the dough into 3 equal parts. Form each into a flat disk, wrap in plastic wrap, and refrigerate for 15 minutes (or up to 2 hours). If chilled longer, let sit at room temperature for 15 minutes before rolling.
• Preheat the oven to 350°F (175°C). Roll one dough disk between two sheets of parchment paper or on a lightly floured surface to 0.125 to 0.25 inch thickness.
• Use a 2-inch round cookie cutter to cut out circles. Place cookies 1 inch apart on a parchment-lined sheet tray.
• Bake for 8 to 9 minutes—avoid overbaking as dark chocolate can taste burnt. Let cool completely on a wire rack.
• While cookies chill, make the filling: In a stand mixer, combine 4 cups powdered sugar, melted butter, and 1 tablespoon vanilla. Add warm water 1 teaspoon at a time until mixture reaches a thick, playdough-like consistency.
• Shape filling into a 2-inch log, wrap in plastic, and refrigerate while cookies bake. Chilling makes slicing easier.
• Once cookies are cool, unwrap filling and slice 0.25 to 0.5 inch thick disks. Place one on the flat side of a cookie, top with a second cookie, and press gently to sandwich.