FOR THE CRUST: Preheat oven to 350°F / 176°C - 30 minutes before baking.Prep quarter-sheet pan: Line pan with foil, building up the sides 1/2" all around. Spray with non-stick spray.

TO MIX THE DOUGH: Use Kitchen Aid mixer, first with whisk, then with paddle. In mixer with whisk, whip egg yolks about 2 minutes on medium speed. Turn speed to low, and slowly drizzle in sugar with mixer running, another 2 minutes. Scrape down bowl and change to paddle. Still on low speed, add soft butter, a chunk at a time with mixer running, 2 more minutes.

Whisk together the flour, baking powder, and salt. Add to the bowl in four batches, pulsing in each addition on the lowest speed, mixing until just barely combined. Scrape up bottom of the bowl and mix in. Transfer dough to floured counter.

Press the dough together and pat into a 6" x 8" rectangle about 1 " thick. Lift onto a quarter-sheet of floured parchment paper. Cover with plastic wrap. Roll out to an even thickness the size of the parchment. Patch corners with excess dough from the sides and ends.

Lift into foil-lined pan, leaving the plastic wrap intact. Smooth the top with the flat side of a plastic bowl scraper.

CHILL DOUGH: For 2 hours in refrigerator or 30 minutes in freezer.

DOCK: Prick the dough through the plastic at 1" intervals, using a wooden skewer, then remove plastic wrap. (If dough is frozen hard, temper at room temperature about 20 minutes, then dock.) Bake partially frozen.

BAKE @ 350°F / 176°C for 20-25 minutes, until a wooden skewer tests clean and the top is very lightly browned. Let cool about 10 minutes before adding the Lemon Curd layer. The idea is to have warm crust and warm filling ready at the same time for baking. If making the crust ahead of time, re-heat before filling.

MAKE THE LEMON CURD FILLING: Peel the zest off the lemons in thin strips with a vegetable peeler, then coarse-chop the zest; it will be strained out of the curd later. Squeeze the lemons and strain the juice; weigh or measure the correct quantity of lemon juice. Caution: Make sure that no white pith gets into the curd mixture, either from peeling the zest, (cut any white pith off the backs of the peeled strips with a pairing knife if necessary), or from juicing the lemons (don't squeeze too close to the pith). Either of these things could cause the curd to be bitter.

Combine the zest, juice, sugar and butter in a heavy saucepan; or, if making the large recipe, use a Dutch oven or rondeau. Bring to a boil over medium heat.

Whisk the yolks in a bowl until smooth. Temper half of the boiling liquid into the yolks, whisking constantly. Pour the yolk mixture back into the pot and insert a probe thermometer into the liquid. Stir constantly in a "figure-8" motion with a wooden spoon or heat-resistant rubber spatula (red handle), going often into the corners of the pot. Continue cooking slowly over medium-low heat until the curd thickens to a firm-set, about 10 minutes overall. During this time, turn the heat on and off as necessary, stirring constantly, to prevent the curd from coming to a boil and to maintain a temperature of about 178°F (81°C). If you don't have a thermometer, cook until you can run your finger through the curd on the back of the spoon and a mark remains; this is called the nappé stage. Remove from heat and Immediately strain through a strainer into a bowl.

TO REPEAT: Cook to between 178° (81°C) and (no higher than) 180° F (82°C), the maximum temperature to keep the curd from coming to a boil. Must cook gently, stirring constantly in a "figure-8" motion. NEVER BOIL.

NOTE: If you have previously made lemon curd on hand, cold from the refrigerator, scale out 550g (1 1/4#); then warm it gently in a bowl set over a pan of simmering water to a temperature of 140°F (60°C) before baking. Stir often.

TO BAKE LEMON CURD BARS: Turn oven down to 275°F (135°C)

Spread warm Lemon Curd on warm crust and bake only 5 minutes just to set and smooth out the curd. Let cool completely in pan. Freeze overnight in pan unwrapped.

TO CUT BARS: Let the pan of frozen bars sit at room temperature for 30 minutes before cutting. Cutting the bars while partially frozen will insure clean sharp edges.

Holding on to the foil, lift the bars out of the pan and place on a cutting board. Peel off the foil and discard, but leave the parchment on the bottom; this will facilitate turning the bars on the cutting board when necessary.

Use a long thin slicing knife and run under hot water (or dip in a pitcher of hot water) and wipe the knife with paper towels after each cut. Use a see-thru ruler for measuring.

FOR 2" SQUARES: Cut crosswise in 2" strips . Cut the strips into 2" squares, starting at the cut end of each strip. Save scraps for snacks.

FOR 1" X 4" LONG THIN BARS: First, cut in half lengthwise. Trim the un-cut side so that each strip is 4" wide. Cut the strips into 1" bars. Save scraps for snacks.