In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
In a large bowl, cream together butter, sugar, and vanilla until fluffy, about 2 minutes. Add eggs and beat until well combined.
Gradually add the dry ingredients, alternating with the sour cream, until well combined.
Cover bowl with plastic wrap and chill for at least 2 hours or overnight.
PREP OVEN AND BAKING SHEETS:
Move the oven rack to the upper-middle and lower-middle positions. Preheat oven to 350 degrees F. Line baking sheets with parchment paper or silicone mats.
ROLL OUT DOUGH:
Knead the dough, about ¼ of the dough at a time, with a small amount of flour. Knead dough until it is not sticky.
Roll dough out onto a floured surface to about ¼ inch thick.
CUT AND BAKE:
Cut rolled dough with cookie cutters and place on prepared baking sheets.
Bake for 10-12 minutes, rotating the baking sheets halfway through baking, or until cookies are very lightly browned.
Cool cookies on the baking sheets for 10 minutes and then transfer to a wire rack until cooled completely.
MAKE FROSTING:
In a medium bowl, combine powdered sugar, corn syrup, clear vanilla extract (regular is fine; clear makes the icing really white), and 2 tablespoons milk.
Whisk together, add more milk a little at a time if needed.
DECORATE COOKIES:
Fit a decorator icing bag with a small round tip and fill with icing.
Decorate as desired with icing, candies, and sprinkles.
Notes
The nutrition information is based on making 5 dozen cookies with this recipe. The nutrition information is just an estimate, because it all depends on how big you make the cookies.
The clear vanilla extract called for in this recipe makes the very white in color. You can also use regular vanilla; note that the icing might not be pure white.
If you want the cookies to have super sharp edges after they're baked, then chill the cut out dough on the baking sheets for 30 minutes before baking them.