Whisk together flour, salt, baking powder, and baking soda in a bowl. Set aside.
Using an electric mixer, cream together butter and sugar. Mix for about 2 minutes.
Mix in egg, vanilla extract and sour cream to butter mixture.
Slowly add in flour mixture and mix until combined. Cover the dough and chill in the refrigerator for about 1 hour.
Preheat oven to 375 degrees.
Lightly dust surface with flour. Dust rolling pin with flour. Roll out dough to about 1/4 inch thick. Using a circle cut out or different shape, cut out dough and place circles on baking sheet lined with parchment paper or silicone mat.
Bake for 7 minutes or until cookie edges just start to turn light brown. Let cookies cool on wire rack.
Stir together powdered sugar and milk in a bowl to make the icing. Once cookies have cooled, top cookies with icing.