Cream the softened butter, granulated sugar, and lemon zest or cinnamon for about 3 minutes, scraping the bowl as needed, then beat in the egg yolk and vanilla. Before adding the yolk, look for a paler mixture with a fluffy texture rather than a dense butter paste.

Whisk the all-purpose flour, almond flour, and salt together, then mix them into the butter mixture only until incorporated. Stop when no dry flour remains so the dough stays tender rather than becoming overworked.

Form two equal portions of dough into discs, cover each with plastic wrap, and refrigerate for about 1 hour. The discs should feel firm throughout before you begin softening them for rolling.

Let the chilled dough soften for 5 to 10 minutes, then roll one disc on a floured surface to about 1/8-inch thick, cut with a 2 1/2-inch round cutter, transfer the rounds to a parchment-lined baking sheet, and reroll the scraps. The dough should remain cold and pliable; if it becomes sticky or difficult to lift, refrigerate it for about 20 minutes until firm before continuing.

Refrigerate the 15 solid rounds for 30 minutes and set the oven to 350°F. The chilled rounds should hold their outlines without soft, sagging edges.

While the bottoms chill, roll and cut 15 rounds from the remaining disc, transfer them to a baking sheet, stamp a center window in each with a small cutter or round piping tip, and refrigerate for 30 minutes. Cutting the windows after transfer keeps the thin rings from stretching as you move them.

Bake the cookies for 12 to 15 minutes, leave them on their pans for 5 minutes, then move them to a rack to cool completely. Take them out when the outer edges first show pale golden brown, while the centers still look light.

Arrange the windowed tops on a cookie sheet and sift confectioners’ sugar over them, then turn the solid bottoms flat side up, spread 1/2 teaspoon jam or lemon curd slightly across each center, and cover with a dusted top. Completely cool cookies keep the sugar dry, and dusting before assembly leaves the fruit windows bright and clear.

Wrap the filled cookies well, keep jam-filled cookies at room temperature when the filling’s storage directions permit, refrigerate curd-filled cookies, or freeze the wrapped sandwiches for longer storage. A close-fitting covering helps protect the delicate crumb from drying out.