How to Make It
Center a rack in the oven and preheat it to 400 degrees F. Butter a 9-inch square baking pan; also butter a piece of aluminum foil that you’ll use to cover the crust.
To make the crust: Put the flour, confectioners’ sugar and salt in a food processor and pulse a couple of times to blend. Scatter the pieces of butter over the dry ingredients and pulse until the butter is cut in coarsely — don’t worry about getting it evenly mixed. Stir the yolk just to break it up and add it a little at a time, pulsing after each addition. When the yolk is in, process in long pulses — about 10 seconds each — until the dough forms moist clumps and curds. Pinch a piece of the dough; it should hold together nicely.
Turn the dough out into the buttered pan. Spread it evenly and, using your fingertips, press the dough down so that you’ve got a compact layer. Don’t worry if it’s bumpy — it’ll be fine. Prick the dough all over with a fork, cover with the foil, buttered side down, and pour in some dried beans and/or rice for pie weights.
Bake the crust for 15 minutes. Carefully remove the foil and weights, return the pan to the oven and bake for another 5 minutes, or until the edges of the crust are golden brown. Place the baking sheet on a rack and let the crust rest while you make the topping. (Leave the oven on.) To make the topping: If there are any stuck-on bits in the processor bowl, wipe them out. Put all of the topping ingredients in the bowl and process, scraping down the sides of the bowl as needed, until you have a smooth mixture, about a minute or two. Rap the bowl against the counter a few times to pop as many of the topping’s bubbles as possible, then pour the topping over the crust.
Bake for 10 minutes, then lower the oven temperature to 325 degrees F and bake for another 25 to 30 minutes (most likely you’ll need the full 30 minutes), or until the topping is set; a tester inserted into the center should come out clean and the topping shouldn’t jiggle when the pan is tapped. Transfer the pan to a rack and let cool until the bottom of the pan feels only just the least bit warm or has come to room temperature. (If you’re the type who likes warm pie, I won’t stop you from cutting the bars sooner.)
Carefully cut the pie bars into quarters. Lift each quarter out of the pan with a broad spatula and cut each quarter into 4 pieces.
Playing Around: Sweet Potato and Marshmallow Pie Bars
If you’d like to cover the top of the bars with toasted marshmallows (and I can’t imagine why you wouldn’t), count on about 30 full-size marshmallows or 2½ cups minis. You may even want to make your own. Just before serving, preheat the broiler. (It’s really best to serve the bars when the marshmallows are still warm, but you can toast them up to 1 hour ahead.) Put the bars, still in their baking pan, on a baking sheet. If you’re using full-size marshmallows, cut them in half or on the diagonal (they’re very pretty cut diagonally) and arrange them cut side down on the sweet potato custard.
If you’re using minis, cover the entire surface of the pie bars with them. Broil for 1 to 3 minutes, until the tops of the marshmallows are golden.
Like sweet potato pie, these bars are best enjoyed the day they’re made. You can make them up to 8 hours ahead, keep them at room temperature and cut them when you’re ready to serve them. Or you can make them up to 1 day ahead and store them, covered, in the refrigerator. Chilled, the topping becomes more like a velvet pudding. It’s a little different from classic pie, but it’s great.
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