1 cup (2 sticks) unsalted butter, softened
3 cups all-purpose flour
1 1/2 cups old-fashioned rolled oats (not quick oats)
1 1/3 cups packed light brown sugar
1 teaspoon salt
1 teaspoon baking powder
1 large egg, separated
1 (14 ounce) can sweetened condensed milk
1/2 cup freshly squeezed lemon juice
2 teaspoons grated lemon zest
2 1/2 cups room-temperature blueberries (about 13 oz.), washed and drained on paper towels
1. Preheat oven to 350°F. Line 9x13-inch metal baking pan with foil, leaving a 1-inch overhang on the ends. Spray foil with cooking spray- bottom and sides of the pan.
2. In a large bowl, whisk together flour, oats, sugar, salt, and baking powder. Using a pastry cutter (or your fingers), blend the butter completely into the flour mixture. Transfer 2 cups of the crumb mixture to another bowl and reserve for the topping.
3. Blend the egg white into the remaining crumbs and then press the mixture into the bottom of the pan to form a level crust. Use the bottom of a flat, wide glass to tap the mixture and even it out.
4. Bake the crust 10-12 minutes, or until it starts to form a dry top.
5. Meanwhile, in a medium bowl, whisk condensed milk, lemon juice & zest, and egg yolk. Let mixture stand for 5 minutes (it will begin to thicken).
7. Sprinkle reserved crumble topping over the lemon-blueberry layer, pressing the streusel between your fingers into small lumps as you sprinkle. Bake until filling is bubbly at the edges and the topping is brown, 25 to 30 minutes.
*The bars may be stored at room temperature for a few hours, but otherwise should be stored in the refrigerator.