Preheat the oven to 350° F.
In a large bowl, cream together butter and powdered sugar; mix until very creamy
Add the 2 whole eggs and 2 yolks only, reserve the whites for later from the egg separation above
Beat until well combined
Add vanilla and salt, blend well
Add flour and baking powder, stir until the dough is very soft
Form a ball with the dough, wrap in plastic wrap and refrigerate for at least two hours
Line a baking sheet with parchment paper
Roll out dough to 1/4" thick on a floured surface and cut out 2 5/8” circles
Brush the circles with the reserved egg whites
Sprinkle each circle lightly with cinnamon sugar and press 5 almond slices into the center of each circle to make a sand dollar
Bake at 350° F for 3 minutes
After 3 minutes, remove the baking sheet from the oven and press the almonds in again, making slight indentations in the circle
Add more cinnamon at this point too, if need be, to adjust the overall coloring of the final cookie.
Place the baking sheet back in the oven and bake until the edges are golden brown, another 10 minutes
Remove from oven let rest on baking sheet for 3 minutes.
Transfer to cooling racks to cool completely