Line 3 completely flat baking sheets with parchment or nonstick baking liners and set aside.
Using a medium-mesh sieve, sift the confectioners’ sugar and almond flour into a large bowl and set aside. In a clean stand mixer fitted with the whisk attachment (or using a large bowl and a hand mixer), whip the egg whites on medium speed until foamy and the wires of the beater(s) leave a trail, 1 to 2 minutes. Add 1 Tbs. of the granulated sugar and continue to whip for another 30 to 45 seconds. Repeat 3 times with the remaining granulated sugar. Once all of the sugar is mixed in, continue whipping the whites until they turn glossy and stiff (when you lift the beater(s) from the bowl, the whites should hold a straight peak that doesn’t curl at the tip), 4 to 8 minutes more.
With a large rubber spatula, fold in half of the confectioners’ sugar mixture. Once most of it has been incorporated, fold in the remaining mixture until just combined.
Using a piping bag fitted with a 1/2- to 3/4-inch round tip (Ateco#806 to #809), pipe the batter onto the prepared sheets in rounds that are about 1 inch in diameter and 1/4 to 1/2 inch thick, spaced about 1-1/2 inches apart. As you pipe, hold the bag perpendicular to the baking sheet and flick the tip of the bag as you finish each cookie to minimize the peaks. Rap the sheet against the counter several times to flatten the mounds and pop any large air bubbles. Let rest until the meringues no longer feel tacky, 20 to 30 minutes. Meanwhile, position racks in the top and bottom thirds of the oven and heat the oven to 325°F.
Put 2 of the cookie sheets in the oven and immediately reduce the temperature to 300°F (let the third sheet sit at room temperature). Bake, rotating the sheets and swapping their positions after 8 minutes, until the meringues are very pale golden, 15 to 20 minutes total. Cool completely on the baking sheets on racks. Meanwhile, return the oven temperature to 325°F and then bake the third sheet as above.
Remove the meringues from the parchment and pair them by size.
Using a piping bag with the same tip used to pipe the cookies, pipe 1 to 1-1/2 teaspoons of the filling onto half of the cookies—you want to use just enough filling that it spreads to the edge when topped but doesn’t squish out much when bitten. Top the filled halves with their partners. The cookies are best the day they’re made, but you can store them in an airtight container at room temperature for up to 1 day or in the freezer for up to 2 weeks.