Cut 4 pieces of parchment paper to the size of your baking sheets.
With a dark marking pen, trace eighteen 2-inch circles on each piece of paper: Trace 4 evenly spaced circles across one end of the parchment, leaving about a 1-inch border of paper.
Trace 3 circles below them, spacing them between the first circles.
Continue with another row of 4, another of 3, and then, a final row of 4 circles.
Turn the paper ink side down; you should be able to see the circles clearly through the paper. Place each piece of parchment on a baking sheet.
Combine the almond flour and confectioners' sugar in a large bowl.
Place the egg whites in the bowl of a mixer fitted with the whisk attachment and whip the egg whites on the highest speed until they begin to hold a shape. Gradually add the granulated sugar and beat until the whites are fluffy and hold a shape. Add the cream of tartar and beat until the whites hold stiff peaks.
Remove the bowl from the mixer and fold in one-quarter of the almond mixture. Fold in the vanilla seeds. Gradually fold in the remaining almond mixture. The meringue may seem dry at first but the two mixtures will combine.
Fit a large pastry bag with a ½-inch plain tip. Place it tip side down in a pitcher or other deep container to keep it upright as you add the batter; fill it with the batter.
Hold the bag upright about ½-inch above the center of a traced circle. Leave the tip in place and pipe the mixture to fill the circle. Lift away the pastry bag and continue to fill all the circles on the first baking sheet.
Lift up the baking sheet with one hand and tap the bottom of the sheet with the other hand to even out the macaroons and smooth out any peaks left by the pastry bag. Repeat with the remaining meringue.
Let the macaroons rest at room temperature for 1 to 2 hours to dry the tops; you should be able to feel a skin over the tops of the macaroons.
Position the racks on the upper and lower thirds of the oven and preheat the oven to 325°F.
If necessary, bake the macaroons in two batches. Bake for 10 minutes, then switch the positions of the baking sheets, turning them around so the macaroons will brown evenly. They will probably need about 10 more minutes of baking: An ideal macaroon has a crispy exterior and a chewy inside; it should be firm to the touch and still soft inside.
Remove the baking sheets from the oven and slide the macaroons, on the parchment paper, to cooling racks. Bake the remaining macaroons. (The unfilled macaroons can be kept in an airtight container for up to 1 week.)
Fit a pastry bag with a ½-inch plain tip and put the buttercream in the bag.
Turn over half the macaroons. Pipe about 1 tablespoon of the buttercream in the center of each inverted macaroon. Top with a second macaroon and press gently to spread the buttercream to the edges of the macaroon.
Filled macaroons are best eaten the day they are made; store in an airtight container at room temperature. (Extras can be individually wrapped and frozen for up to 2 weeks.)