Preheat and Prepare: Preheat your oven to 300°F (150°C). Line two large baking sheets with parchment paper or silicone baking mats to prevent the cookies from sticking.
Whip the Egg Whites: In a clean glass or metal bowl, beat the egg whites until they form soft peaks. Gradually add the sifted powdered sugar one tablespoon at a time while continuing to beat. Keep whisking until the mixture is thick, glossy, and forms stiff peaks.
Reserve the Glaze: Remove about 1/2 cup of the stiff egg white and sugar mixture and set it aside in a small bowl. Stir in the lemon juice; this will be the white icing for the top of your stars.
Create the Cookie Dough: Gently fold the ground almonds and the ground cinnamon into the remaining egg white mixture. Mix until a stiff, slightly sticky dough forms. If the dough is too wet, you can add a tablespoon more of ground almonds.
Roll and Cut: Dust your work surface lightly with powdered sugar. Roll the dough out to about 1/2-inch thickness. Use a star-shaped cookie cutter to cut out the cookies. To prevent sticking, dip the cutter in powdered sugar between each cut.
Glaze the Stars: Place the stars on the prepared baking sheets. Using a small spoon or a pastry brush, carefully spread a thin layer of the reserved glaze over the top of each cookie, reaching all the way to the points of the stars.
Bake to Perfection: Bake for 12 to 15 minutes. You want the icing to remain white and the cookies to just set. Do not let the glaze brown. The cookies will still feel soft but will firm up as they cool.
Cool Thoroughly: Allow the cookies to cool on the baking sheet for 10 minutes before transferring them to a wire rack to cool completely. This ensures the base sets properly.