Preheat oven to 300 degrees. Sift confectioners' sugar into a bowl. Whisk in almonds; set aside. Line 2 baking sheets with parchment paper or nonstick baking mats (such as Silpats), and mark circles using a 1 1/2-inch cutter dipped in flour.
Put egg whites in the bowl of an electric mixer fitted with the whisk attachment. Beat on medium speed until foamy, then beat in salt. Beat in granulated sugar, 1 teaspoon at a time, until medium-soft peaks form. Transfer mixture to a large bowl.
Using a rubber spatula, fold half the almond mixture into the egg white mixture until just incorporated. Fold in vanilla and remaining almond mixture until just incorporated. Firmly tap bottom of bowl on counter to eliminate air pockets.
Transfer mixture to a pastry bag fitted with a 1/2-inch plain tip (such as Ateco #806). Pipe mixture into marked circles on prepared baking sheets. Bake, rotating sheets halfway through, until macarons are slightly firm and can be gently lifted off parchment (bottoms will be dry), 20 to 25 minutes. Let cool on sheets 5 minutes. Transfer macarons on parchment to a wire rack; let cool completely.
Spread 2 teaspoons buttercream on flat sides of half the macarons, then sandwich with remaining halves, keeping flat sides together. Refrigerate until firm, about 20 minutes, before serving.