Whisk together flour, salt, baking soda, and baking powder in a medium bowl. Set aside.
Using stand mixer, mix together butter and sugar. Add vanilla extract and eggs (one at a time).
Mix in ½ the sour cream, then ½ the flour. Repeat with remaining ingredients.
Divide dough in half, then shape each half into a ball. Flatten balls into disks about 1-inch thick. Wrap disks in plastic wrap, then chill for at least 2 hours (or overnight).
Preheat oven to 375°F (190°C). Lightly spray two baking sheets with nonstick spray.
Using rolling pin, roll out one disk of dough about ¼-inch thick on a lightly floured board.
Use cookie cutters to cut dough, then transfer cookies to baking sheet, spacing at least 1-inch apart.
Bake cookies for about 8 minutes, or until cookie bottoms are golden brown. Let cookies cool on pan for 5 minutes, then transfer to a baking rack to cool completely.
Once cookies have cooled, decorate with frosting and sprinkles, or icing. Enjoy!
Prepare Frosting:
Using stand mixer, mix butter until smooth. Add sour cream, vanilla extract, and salt.
Mix in powdered sugar until smooth. Add food coloring, if desired.
If frosting is too thin, add more powdered sugar until frosting reaches desired consistency. If frosting is too thick, add additional milk (a drop or two at a time), until frosting reaches desired consistency.
Spread frosting over cookies with a butter knife or small offset spatula. Add sprinkles, if desired.
Prepare Icing:
Combine powdered sugar, milk, and vanilla extract in a small bowl, mixing until smooth. Add food coloring, if desired.
For thick icing (for lines), you will need about 1 to 1 ½ tablespoon milk. The consistency should be barely pourable. For thinner icing (used to flood cookies), you will need about 1 ½ to 2 tablespoons milk. Start with the smaller amount of milk, then add more (a few drops at a time) to reach desired consistency. If icing gets too thin, just add more powdered sugar.
Transfer finished icing to decorating bulbs or squeeze bottles. Outline cookies with thick icing first, then fill in with thinner icing.
Let icing harden before storing cookies. It can take up to 24 hours (chill cookies in refrigerator to speed up process).
Notes
Read information before recipe for additional tips (especially regarding decorating cookies).Bake one sheet of cookies at a time for the best results.Recipe makes about 4 to 5 dozen cookies, but the amount you get will depend on how thick you roll the dough, how big your cookie cutters are, how closely you cut the cookies, etc.Icing ingredients are for one batch of icing. Make one thick batch for lines (or more, if you'd like colored lines), then as many thin batches as desired for each color you would like to use to fill in the cookies.