In a large bowl, cream the butter and powdered sugar for at least 3 minutes until light and fluffy.
Add in the 2 whole eggs, 2 egg yolks, and vanilla. Blend until well combined.
Slowly blend in the salt, flour, and baking powder (remember to scrape down the sides of the bowl) until a soft dough forms.
Divide the dough in half and wrap each half in plastic wrap.
Chill the dough for at least 20 minutes.
Preheat oven to 350 degrees Fahrenheit (175 Celsius)
Roll the chilled dough to approximately ⅛" thickness on a lightly floured surface.
Use a 3" round biscuit or cookie cutter to cut circles out of the dough.
Place the circles onto parchment-lined baking sheets.
Re-roll the dough scraps and continue rolling and cutting circles.
Continue the same process with the other half of the frozen dough.
In a small bowl, mix the granulated sugar and cinnamon.
Use a pastry brush to brush the cut cookies with the beaten reserved egg whites.
Place 5-6 almond slices (in a star pattern) on the top of each cookie and liberally sprinkle each cookie with the cinnamon and sugar mixture.
Bake the cookies in a preheated oven for 10 minutes.
Rotate the cookie sheets and continue baking for an additional 10 minutes.
Remove from the oven and cool on a wire rack.