Preheat your oven to 250°F (120°C) and prepare your baking sheets with silicone liners or parchment paper.

Combine ½ cup of confectioners' sugar, 10 ounces of sliced almonds, and ground cinnamon in a food processor and pulse until finely ground.

Beat the egg whites in a clean, dry bowl until soft peaks form, then gradually add the remaining confectioners' sugar until thick and creamy.

Gently fold the almond mixture and lemon zest into the whipped egg whites using a spatula to combine.

Roll the stiff dough between parchment paper to ¼ inch thickness and cut out star shapes with a cookie cutter.

Spread a thin layer of reserved egg white mixture over each star-shaped cookie and sprinkle additional sliced almonds on top.

Bake for about 30 minutes until the bottoms are light golden brown, then turn off the oven and leave cookies inside for an additional 10 minutes.