In a large bowl, cream together butter and powdered sugar, until very creamy.
Add 2 whole eggs, plus 2 additional 2 egg yolks; reserve the whites for later.
Beat mixture until well combined, then mix in the vanilla and salt.
Stir in the flour and baking powder until the dough is soft.
Form the dough into a big ball, wrap in plastic, and refrigerate for about 2 hours.
Preheat the oven to 350° F and line a baking sheet with parchment paper or non-stick silicone baking mat.
Flour your workspace and roll out the cookie dough to 1/4" thickness. Use a round cookie cutter to cut the dough into circles. We used a 2 5/8” cookie cutter, but anything between 2 1/2" and 3" should work.
Brush the tops of the cookies with the reserved egg whites and sprinkle lightly with cinnamon sugar.
Press 5 almond slices into the center of the circle in a star pattern like you'd see on a sand dollar.
Bake at 350° F for 3 minutes.
At the 3 minute mark, remove the baking sheet from the oven and press the almonds in a bit further. Use caution as the cookies will be hot. You may want to use a chopstick or something similar for this step.
Return to oven and bake 10 more minutes, until golden brown.
Remove from oven and allow cookies to rest on the baking sheet for 2-3 minutes, then transfer to a wire rack to finish cooling.