In the bowl of a stand mixer fitted with the paddle attachment, combine butter and sugar. Add egg yolks one at a time and continue mixing. Add almond extract and salt, followed by flour. Mix until cookie dough is well combined. Refrigerate cookie dough for 1 hour.
Line three baking sheets with parchment paper and preheat oven to 375 degrees F. Scoop 1 tablespoon of chilled cookie dough and roll into a ball. Roll cookie dough balls in Christmas sprinkles, then use a teaspoon to gently create thumbprint.
Bake for about 10 minutes or until bottoms of thumbprints are a light golden brown. Remove from oven and while cookies are still hot, use teaspoon to press thumbprint down again if it has puffed up. Let cool completely.
When cookies have cooled, prepare buttercream. In the bowl of a stand mixer, beat butter and powdered sugar together until well combined. Add vanilla extract and pinch of salt. Add 1-2 tablespoon heavy cream until frosting is smooth. Pipe onto cooled cookies with a star tip.