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 ¼" thick on a floured surface and cut out 2 ⅝” 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 the oven and let them rest on a baking sheet for 3 minutes.
Transfer to cooling racks to cool completely.