In the bowl of stand mixer fitted with the paddle attachment, or in a large bowl with an electric mixer, beat the butter and sugar until light and fluffy.
Mix in sour cream, eggs and vanilla until thoroughly incorporated. The mixture look curdled, but it will come together once the dry ingredients are incorporated.
In a medium bowl, whisk together the flour, salt, baking soda. Gradually add the dry ingredients to the butter mixture and mix until combined. Cover and chill for at least 6 hours or overnight.
When you are ready to bake, preheat oven to 350°F and line baking sheets with parchment paper.
Use a small cookie scoop (2 teaspoons) to portion dough onto prepared cookies sheets 2 inches apart.
Bake in preheated oven for 9-10 minutes. Allow cookies to cool on cookie sheets for 5 minutes before removing to a wire rack to cool completely.
For the Buttercream:
In the bowl of a stand mixer fitted with the paddle attachment, or in a large mixing bowl with an electric mixer, cream butter on medium-high speed for about 6 minutes. With the mixer on low speed, slowly add in confectioners’ sugar, and continue beating until well blended.
Add in vanilla, and 1 tablespoon of heavy cream or milk. Blend on low speed until moistened.
Add an additional 1 to 2 tablespoons of heavy cream or milk until you reach the desired consistency. Beat at high speed until frosting is smooth and fluffy; about 5 more minutes.
Pipe or spread frosting onto cooled cookies and top with festive sprinkles.