Preheat the oven to 350 F. Line two baking sheets with parchment paper. For the best results, I recommend using non-insulated, light-colored baking sheets.

In a medium bowl, whisk or sift together the flour, baking soda, and salt. Set aside for a moment.

In the bowl of a large stand mixer fitted with a paddle attachment, combine the butter and sugar. Beat on medium speed for 2 minutes, until light and fluffy.

Add the egg, vanilla, sour cream, and milk, and mix until well combined. It is normal for it to look a bit broken at this point - don't worry! It all comes together when the flour is added.

Turn the mixer speed to low, and gradually add the flour mixture. Mix until just a few streaks of flour remain. Do not overmix here- stop the mixer just when the flour is incorporated. After you stop the mixer, give the dough a few good stirs by hand with a rubber spatula, being sure to scrape the bottom and sides of the bowl so everything is well-mixed.

Use a #24 cookie scoop (3 TBSP, or 1.75 oz) to scoop large balls of cookie dough —you should get about 18 cookies from this recipe. Roll the balls between your hand and place them a few inches apart on the parchment-covered baking sheet.s

Bake at 350 F for 16-18 minutes, until the cookies are puffed and just starting to take on color around the edges. The tops should still remain virtually colorless.

Let the cookies cool completely before frosting.

Beat the butter with an electric mixer fitted with a whisk attachment on medium speed for 2-3 minutes, until creamy and light in color.

Add the powdered sugar, milk, vanilla, and salt, and mix well, until light and fluffy.

If the frosting is too stiff for your liking, you can add more milk, a bit at a time, until you get a texture you like. If it is too soft, add a few spoonfuls of powdered sugar to adjust the texture.

Put a generous dollop of frosting on top of each cookie, and spread it around evenly over the top. Finish with a big pinch of sprinkles or other decorations.

Store the cookies in an airtight container. They can be kept for about a week, but the taste and texture is best if enjoyed within 3-4 days.

The cookies and frosting can both be made in advance, and the cookies can be stored in the freezer for up to 4 months, while the frosting can be frozen for 2 months or refrigerated for 2 weeks. Make sure to store it with plastic pressed tightly to the top so it doesn’t form a crust. Allow it to come to room temperature and re-whip before using.