Blueberry Streusel Bars with Lemon-Cream Filling

makes 24

1 cup (8-ounces) unsalted butter, softened; plus more for the pan

3 cups (13 1/2-ounces) unbleached all-purpose flour

1 1/2 cups old-fashioned rolled oats (not quick cooking)

1 1/3 cups firmly packed light brown sugar

1/8 teaspoon ground cinnamon

1 teaspoon kosher salt

1 teaspoon baking powder

1 extra large egg, separated

1 (14-ounce) can sweetened condensed milk

2 teaspoons grated lemon zest

1/2 cup freshly squeezed lemon juice

1/2 teaspoon pure vanilla extract

18 ounces room temperature blueberries, washed and dried

2 teaspoons cornstarch

Position a rack in the center of the oven and preheat to 350º F.  Line a 9 x 13-inch metal baking pan with foil, leaving a 1-inch overhang on the ends.  Lightly butter the sides and bottom with butter.

In a large bowl, combine the flour, oats, sugar, cinnamon, salt, and baking powder. Using your fingers, blend the butter completely into the flour mixture.  Transfer 2 cups of the crumb mixture into another bowl.  Blend the egg white into the remaining crumbs, then press the mixture into the bottom of the pan to form a level crust.  Bake the crust until the top starts to form a dry top, about 10-12 minutes.

Meanwhile, in a medium bowl whisk the condensed milk, zest, lemon juice, vanilla, and egg yolk.  Let the mixture stand for 5 minutes, it will begin to thicken.  In a separate bowl, toss the blueberries with the cornstarch.

Remove the crust from the oven and sprinkle the blueberries on top.  Drop spoonfuls of the lemon cream mixture over the blueberries.  Spread gently to distribute more evenly.  Bake until the lemon cream just begins to form a shiny skin, about 7-8 minutes.

Sprinkle the reserved topping over the blueberry-lemon layer, pressing the streusel between your fingers into small lumps you sprinkle.  Bake until the filling is bubbling at the edges and the topping is golden brown, about 25-30 minutes.

Let the bars cool in the pan on a wire rack until just warm, about an hour.  Carefully lift them out of the pan using the foil overhang and return it to a wire rack to cool completely.  Return the bars to the pan, cover and refrigerate until chilled.  When ready to serve, remove the bars from the pan.  Remove the foil and cut into 24 bars.  Serve chilled or at room temperature.  Enjoy!

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