In a mixing bowl, combine 1 cup of softened unsalted butter and 1 cup of powdered sugar. Cream them together using an electric mixer on medium speed until light and fluffy for about 3-5 minutes.

Beat in 2 whole eggs and 2 egg yolks to the butter-sugar mixture followed by 1 teaspoon of vanilla extract and a pinch of kosher salt. Mix until smooth.

Gradually add 3 cups of all-purpose flour mixed with 1 teaspoon of baking powder into the wet mixture. Combine until no flour streaks remain.

Cover the bowl with plastic wrap and refrigerate for at least 30 minutes.

Preheat the oven to 350°F (175°C) and roll out the dough to about ¼-inch thickness. Use a round cookie cutter to cut out circles.

In a small bowl, whisk an egg white and brush it over the tops of each cookie. Sprinkle with cinnamon sugar and decorate with sliced almonds.

Bake for 8-10 minutes, or until the edges are lightly golden brown.

Remove cookies from the oven and let them cool for about 5 minutes before transferring to a wire rack.