(1/2 cup) chilled, unsalted butter, cut into 1/2-inch cubes
(1/2 cup) chilled vegetable shortening, cut into 1/2-inch pieces
5 to 6 Tbs.cold milk; more as needed
Put the flour, sugar, salt, and baking powder in a food processor fitted with a steel blade. Chill for 20 to 30 minutes.
Pulse the dry ingredients together for a few seconds to blend. With the processor off, Add half of the butter and half of the shortening. Pulse 5 times and then process for 5 seconds. Add the remaining butter and shortening and pulse again 5 times, then process for 5 seconds. You should have a mixture of both large and small crumbs. Empty the mixture into a large mixing bowl.
Drizzle 1 Tbs. of the cold milk around the edge of the bowl, letting it trickle into the crumbs. Flick the moistened crumbs toward the center with a table fork, rotating the bowl as you work. Repeat with the remaining 4 Tbs. cold milk, 1 Tbs. at a time. As you add the water, the crumbs should begin to form larger clusters. Once you've added 5 Tbs. milk total, take a handful of crumbs and squeeze them gently; they should hold together. If they fall apart easily, the mixture needs more milk: add the remaining Tbs., one tsp. at a time, checking the consistency each time you add more milk. If the crumbs still fail to hold together, you can add additional milk, but do so sparingly.
Gather a handful of the crumbly dough and press it against the side of the bowl to form a small mass, flouring your hand as needed to prevent excessive sticking. Increase the size of this mass by pressing it into more of the crumbly mixture until you've used up about half of the total mixture in the bowl. Make a second mass of dough with the remaining crumbs. If some of the crumbs on the bottom of the bowl need more moistening, add a few drops of milk.
Form the two masses of dough into balls, dust them with flour, and flatten them into 4- to 5-inch disks. Pat the disks to release any excess flour. Score the tops lightly with the side of your hand to create a tic-tac-toe pattern. With cupped hands, rotate each disk on the work surface to smooth the edges of the disks. Wrap each in plastic wrap. Chill at least 30 minutes before using.