In a medium bowl, whisk together the flour, baking powder, and salt. Set aside
In a large bowl using a hand held mixer or a stand mixer fitted with a paddle attachment, beat together the softened butter and sugar on high speed for about 3 minutes or until completely smooth and creamy. The butter should be a pale yellow color.
Add in the egg, and vanilla and continue to beat on high speed until combined, about 1 minute. Make sure to scrape down the sides of the bowl when needed.
Add the dry ingredients (flour, baking powder, and salt) to the wet ingredients and mix until evenly combined. The dough will be very soft but should not stick to your hands. If you find that the dough is too sticky and will be difficult to roll out with a rolling pin, add in 1 more tablespoon of flour until it is not longer sticky to your hands but still very soft.
Transfer dough on a lightly floured silicone baking mat or parchment paper and with a rolling pin, roll out the dough about 1/4-inch thick. Place the silicone mat with the rolled out dough in a baking sheet. Cover baking sheet with aluminum foil and refrigerate for at lease 4 hours, up to 2 days. Overnight refrigeration is best.
Preheat oven to 350°F .
Once the cookies have chilled for at least 4 hours, cut out shapes of your choice. Any left over dough can be re-rolled to make more cookies. Do not roll the dough too many times to prevent warming the dough.
Spread cookies about 1½" apart on a baking sheet lined with parchment paper. Bake cookies for 12-13 minutes or until the edges are a very light brown. The center of the cookies should still be very soft to touch but that will harden as the cookies cool.
Remove cookies from oven and let them sit in baking sheet for 5 minutes. Then, transfer cookies onto a cooling rack and let them cook completely.
Decorate cookies once they are completely cooled.