Working quickly, cut the stick of cold butter in half long-wise and then cut the 2 halves into 8 even pieces. You should have 16 pieces of butter. Wrap half the butter in plastic wrap and put in the refrigerator. Wrap the other half of butter and put it in the freezer. Allow the butter to chill/freeze for at least 30 minutes.
Mix the flour, salt, and baking powder, put in a plastic bag, and also put in the refrigerator for 30 minutes. (Cold ingredients equal flakier pie crust!)
Fill a 1 cup liquid measuring cup with ice and add water to fill to the top. Put back in refrigerator as well.
Add flour mixture and the refrigerated butter to the bowl of the food processor and blitz for 5-second bursts until the mixture is evenly course and crumbly. Add the frozen butter and blitz for 2-second bursts until the frozen butter is the size of peas. Add 2 ½ Tbsp of the ice water and the vinegar and blitz for 6 1-second bursts. Stop and pinch the dough between your fingers. If it doesn't hold together, add another ½ Tbsp of water, pulse 3 times, and do the pinch test again. Repeat until the dough holds together when pinched.
Working quickly (you want the butter to stay cold), transfer the crumbly dough into a gallon-size plastic bag. Keeping the bag open, carefully knead the dough from the outside just until it holds together. (This method keeps your hands from warming the dough too much.) Shape the dough into a disc, wrap in plastic wrap, and refrigerate for at least an hour, preferably overnight.
*If you don't have a food processor, you can make the crust by hand using a pastry cutter and repeating all the steps. I prefer the food processor method because it's faster and thus, easier to keep the butter cold, yielding a flakier pie crust.