In a large bowl, whisk together flour, baking powder, baking soda and salt. Set aside.
With a hand mixer or stand mixer fitted with the paddle attachment cream together sugar and butter until light and fluffy, about 3 minutes. Mix in egg and vanilla. Add sour cream and mix just until combined. Add dry ingredients to bowl and mix to combine.
Transfer dough to a large piece of plastic wrap on a work surface and wrap to cover dough completely, shaping into a disc. Chill dough at least 2 hours or overnight.
When you're ready to bake:
Preheat the oven to 375˚F.
Lightly grease 2 large baking sheets or line with parchment paper. (see recipe note #1)
Divide dough in half. Place one half back in the refrigerator; transfer other half to lightly floured work surface. Roll dough ¼" thick (no thinner!) - cut with desired cookie cutters and transfer to prepared baking sheets, leaving at least 1" between cookies as they'll puff and spread.
Bake 8 to 10 minutes until set, but look slightly underdone (this is key to the cookies staying soft). Remove from oven and carefully transfer to a cooling rack. Repeat rolling, cutting and baking process with remaining refrigerated dough.
I like to have 2 baking sheets to work with so that each batch of cookies starts on a cool sheet. Alternating between sheets works well for 2 batches or 4 or more, which I often make for edible gift giving during the holidays.
If you double this recipe, work with half of the dough at a time. Keeping the second half refrigerated until you've cooked all of the cookies from the first batch.
When using Sugar Cookie Icing, be sure you give it ample time to cool and harden before stacking the cookies on a platter or in a gift box.
Note that the nutrition information for these soft sugar cookies includes a full recipe of Sugar Cookie Icing.