In a medium bowl, whisk the flour, baking soda, and salt to combine.
In a large bowl, cream the butter and sugar together with mixer until light and fluffy, about 5 minutes.
Add the egg and mix until only incorporated.
Next, mix in the sour cream and vanilla and mix until creamy.
Add the flour ¼ cup at a time and only mixing until combined.
On a lightly floured surface, add the dough and form into a disc and wrap in plastic wrap. Chill for at least 1 hour in the refrigerator.
Preheat the oven to 300˚F.
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 your favorite cookie cutter or the rim of a glass for round cookies.
Transfer the cookies to a parchment lined 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.