Prepare No-Fail Tart Crust as directed, except in Step 1 process 1 cup white cheddar with flour and salt for two to three pulses, then add butter.
Keep oven temperature at 350 degrees F. On a foil-lined baking pan toss sweet potatoes with half the olive oil, salt, and black pepper. Repeat with squash on a second foil-lined baking pan. Roast 20 minutes or until tender, rotating pans once halfway through. Let cool 5 minutes. Sprinkle remaining 1/2 cup white cheddar over bottom of the baked crust. Arrange sweet potatoes and squash in rows over cheese. Bake 10 minutes; cool 20 minutes.
Meanwhile, in a large skillet cook pancetta over medium-high heat until browned and crisp; set aside. In small bowl stir together the butter, chili powder, and brown sugar. Just before serving, brush butter mixture over tart. Sprinkle with pancetta and sage leaves. Serve warm.
No-Fail Tart Crust
Preheat oven to 450 degrees F. (For nonstick pans preheat oven according to instructions on package.) In a food processor pulse flour, butter, and salt 10 to 20 seconds or until mixture looks like fine crumbs. Do not overprocess.
In a small bowl whisk together yolks and the water. With processor running, add yolk mixture. Stop as soon as dough just starts to come together, about 5 seconds.
Crumble to distribute evenly in an 118-inch rectangular tart pan or 11-inch round pan with removable bottom; press into pan. (Or gather dough into a ball; flatten. Roll out dough between two pieces of parchment paper into a 129-inch rectangle. Transfer dough to pan. Press into corners; trim excess.)
Using a fork, prick dough all over. Line pastry with a double thickness of foil. Bake 12 minutes. Reduce temperature to 350 degrees F. Remove foil. Bake 8 minutes more or until pastry is golden. Let cool completely. Use baked crust to prepare seasonal tarts.