1. To make the macarons: Preheat the oven to 350 degrees. In a medium bowl, whisk together confectioners' sugar and ground almonds. In the bowl of an electric mixer fitted with the whisk attachment, whip egg whites with salt on medium speed until foamy. Increase speed to high and gradually add granulated sugar. Continue to whip until stiff glossy peaks form. With a rubber spatula, gently fold in the confectioners' sugar mixture until completely incorporated.

2. Line baking sheets with parchment paper; set aside. Fit a pastry bag with a 3/8-inch #4 round tip, and fill with batter. Pipe 1-inch disks onto prepared baking sheets, leaving 2 inches between cookies. The batter will spread a little. Let stand at room temperature until dry, and a soft skin forms on the tops of the macarons and the shiny surface turns dull, about 15 minutes.

3. Bake, with the door of the oven slightly ajar, until the surface of the macarons is completely dry, about 15 minutes. Remove baking sheet to a wire rack and let the macarons cool completely on the baking sheet. Gently peel off the parchment. Their tops are easily crushed, so take care when removing the macarons from the parchment. Use immediately or store in an airtight container, refrigerated for up to 2 days or frozen for up to 1 month.

4. To fill the macarons: Fill a pastry bag with the filling. Turn macarons so their flat bottoms face up. On half of them, pipe about 1 teaspoon filling. Sandwich these with the remaining macarons, flat-side down, pressing slightly to spread the filling to the edges. Refrigerate until firm, about 1 hour.

5. Variations: To make coffee-flavored macarons: In step 1, add 2 drops brown food coloring to the egg whites after they are whipped. In step 4, blend 1/2 cup macaron filling with 1 1/2 teaspoons espresso powder dissolved in 1/2 teaspoon warm water for the filling. To make cassis-flavored macarons: In step 1, add 2 drops purple food coloring to the egg whites after they are whipped. In step 4, use 1/3 cup good-quality cassis jam for the filling. To make pistachio-flavored macarons: In step 1, add 2 drops green food coloring to the egg whites after they are whipped. In step 4, combine 1/2 cup macaron filling with 1 tablespoon pistachio paste for the filling.