In a medium bowl, whisk the flour, baking soda, and salt to combine.
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.
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.
Preheat the oven to 300˚F (150˚C). Line a baking sheet with parchment paper.
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.
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.
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.
Frost the cooled cookies and decorate as you wish.
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