Pulse confectioners' sugar and almond flour in a food processor until combined. Sift mixture 2 times.
Preheat oven to 375 degrees. Whisk whites with a mixer on medium speed until foamy. Add cream of tartar, and whisk until soft peaks form. Reduce speed to low, then add superfine sugar. Increase speed to high, and whisk until stiff peaks form, about 8 minutes. Sift flour mixture over whites, and fold until mixture is smooth and shiny.
Transfer batter to a pastry bag fitted with a 1/2-inch plain round tip, and pipe 3/4-inch rounds 1 inch apart on parchment-lined baking sheets, dragging pastry tip to the side of rounds rather than forming peaks. Tap bottom of each sheet on work surface to release trapped air. Let stand at room temperature for 15 minutes. Reduce oven temperature to 325 degrees. Bake 1 sheet at a time, rotating halfway through, until macarons are crisp and firm, about 10 minutes. After each batch, increase oven temperature to 375 degrees, heat for 5 minutes, then reduce to 325 degrees.
Let macarons cool on sheets for 2 to 3 minutes, then transfer to a wire rack. (If macarons stick, spray water underneath parchment on hot sheet. The steam will help release macarons.)
Sandwich 2 same-size macarons with 1 teaspoon filling. Serve immediately, or stack between layers of parchment, wrap in plastic, and freeze for up to 3 months.
Piping the perfect macaroon takes a little practice. Treat it as you would a rosette, bringing the pastry tip to the side of the circle, rather than forming a peak, to finish.
Chocolate: Substitute 3 tablespoons unsweetened Dutch-process cocoa powder for 1/4 cup of the almond flour.