How to Make It
Prepare the Cookie Base: Preheat oven to 350°F. Grease an 8-inch square baking pan with butter. Line pan with parchment paper, allowing paper to extend past edges of pan. Grease parchment. Pulse flour, powdered sugar, and salt in a food processor until combined, about 3 times. Add butter; pulse until mixture resembles coarse sand, 6 to 7 times. Press flour mixture into prepared pan. Bake in preheated oven 15 minutes.
Prepare the Filling: Combine granulated sugar, lemon juice, cornstarch, and salt in a medium saucepan; bring to simmer over medium-high. (Do not bring to a boil.) Lightly beat eggs and yolks in a medium bowl. Add 1/2 cup of the hot sugar mixture to eggs, whisking constantly. Gradually add egg mixture to remaining sugar mixture in saucepan, whisking constantly. Cook, whisking constantly, until thick and bubbly, about 2 minutes. Remove from heat. Stir in butter until melted and combined. Spread hot filling evenly over prepared cookie base.
Prepare the Topping: Pulse flour, powdered sugar, lemon zest, and salt in a food processor until combined, 5 to 6 times. Add butter; pulse until mixture forms pea-size pieces, 4 to 5 times. Add oats; pulse until combined, 2 to 3 times. Using your hands, press the mixture together into small clumps. Sprinkle clumps evenly over filling, pressing lightly to adhere. Bake at 350°F until top is lightly browned, 35 to 40 minutes.
Cool completely in pan. Lift Lemon Crumble from pan using parchment as handles. Trim the edges, if necessary, and cut into 16 bars.