FOR THE COOKIES: – 1 & 3/4 cups (210g) all-purpose flour – 1/2 cup (42g) unsweetened cocoa powder, sifted – 1/2 teaspoon baking soda – 1/2 teaspoon salt – 1/2 cup (113g) unsalted butter, softened – 1/2 cup (100g) granulated sugar – 1/2 cup (100g) firmly packed light brown sugar – 2 large egg – 1 teaspoon vanilla extract – 3/4 cup (127g) semisweet chocolate chip FOR THE FILLING: – 1/2 cup (113g) unsalted butter, softened – 1/2 cup (55g) confectioners' sugar, sifted – 6 ounces (170g, about 1 cup) bittersweet chocolate (about 70% cacao), melted and cooled – 1/2 teaspoon vanilla extract