In a medium bowl, whisk the flour, baking soda, and salt to combine.
2. Step
In a large bowl, cream the butter and sugar together with an electric hand mixer until light and fluffy, about 5 minutes. Add the egg and mix until fully incorporated. Add the sour cream and vanilla and mix until creamy. Add the flour mixture, a third at a time, until fully combined.
3. Step
Lightly flour a work surface and turn out the dough. Press the dough together into a disc and wrap in plastic wrap. Chill for 1 hour in the refrigerator.
4. Step
Preheat the oven to 300˚F (150˚C). Line a baking sheet with parchment paper.
5. Step
Remove the plastic wrap from the dough and lightly flour a work surface. Roll out the dough to about ½-inch thick. Cut out circles using a glass or cookie cutter.
6. Step
Transfer the cookies to the prepared baking sheet, leaving about 1 inch in between cookies. Bake for 8 minutes, until the cookie bottoms are golden brown and the tops are pale. Let the cookies cool on a wire rack.
7. Step
Make the frosting: in a large bowl, beat the butter with an electric hand mixer until fluffy. Sift in the powdered sugar and mix until incorporated. Add the milk and continue to beat until the frosting is smooth and velvety. Add food coloring, if desired.
8. Step
Frost the cooled cookies and decorate as you wish.
