In a medium bowl, whisk together the flour, baking soda, and salt. Once combined, set the bowl aside for later. In a large mixing bowl, add the room temperature butter and granulated sugar. Using an electric mixer, beat them together on medium speed for about 2 minutes.

Next, add the room temperature egg, vanilla extract, sour cream, and milk to the butter and sugar mixture. Continue mixing until everything is well combined. Don’t worry if the dough looks slightly broken at this stage—it will come together as you proceed.

Turn the mixer speed to low and gradually add the flour mixture, a little at a time. Mix until just a few streaks of flour remain. Stop the mixer and finish stirring by hand using a spatula to ensure any stray flour at the bottom and sides of the bowl is fully incorporated.

For the best results, use a light-colored, non-insulated baking sheet. Insulated cookie sheets can cause the cookies to spread too much, so stick with a good-quality aluminum sheet. Line the sheet with parchment paper.

Use a cookie scoop that holds about 1/4 cup of dough to portion out the cookies onto the prepared baking sheet. Leave a few inches of space between each cookie. For smoother tops, gently roll the dough into balls before placing them on the sheet.

Preheat your oven to 350°F (175°C). Bake the cookies for 16-18 minutes, or until they are set and just beginning to turn golden around the edges. Let them cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.

Start by beating the room temperature butter on medium-high speed for about 3 minutes until it’s light and creamy. Add the salt and flavoring (vanilla or Princess flavoring). Then, mix in the milk. Gradually add the powdered sugar, mixing on low speed until it’s fully incorporated. Increase the speed and beat until the frosting is smooth and fluffy. Adjust the consistency by adding more milk if it’s too stiff or more powdered sugar if it’s too soft.

Once the cookies are completely cool, it’s time to frost them. Place a generous dollop of frosting on each cookie and spread it evenly over the top using a mini offset spatula. To create a classic bakery swirl, give the spatula a little twist of the wrist as you finish spreading.

Finally, add a big pinch of sprinkles or sparkling sugar on top of the frosting to give your cookies that extra touch of sweetness and visual appeal.