1 cup unsalted butter, softened
3 cups flour
1 1/2 cups old-fashioned oats
1 1/3 cups brown sugar
1 teaspoon salt
1 teaspoon baking powder
2 eggs, separated
2 cans (14 ounce, each) sweetened condensed milk
1/2 cup fresh lemon juice (Really, squeeze the lemon. The better flavor is worth it.)
2 tablespoons grated lemon zest
3-4 cups fresh blueberries
Heat oven to 350.
Line a 9x13 pan with foil, and butter the bottom and sides of foil
Combine flour, oats, sugar, salt, and baking powder.
Blend the butter into the flour mixture to form a crumb mixture.
Transfer 1 1/2 cups crumb mixture to another bowl and reserve for topping.
Bake 10-12 minutes until it forms a dry top.
Meanwhile, in another bowl, whisk the condensed milk, lemon juice, lemon zest, egg yolk, and remaining egg.
Let stand for 5 minutes to thicken slightly.
When the crust is baked, pour filling over in an even layer.
Sprinkle blueberries evenly on top of the filling layer.
Sprinkle reserved crumble topping over the blueberries.
Bake for 30-40 minutes until the topping is brown and filling has set (bars won't jiggle when pan is moved).
Allow to cool completely before cutting into bars. (If you are impatient, the filling will ooze and the crumble will crumble even more than normal.)
(Yield: 24 bars)