Old-Fashioned Blueberry Crumb Bars

This recipe has been floating in the ether for years. I first saw these bars on the wonderful blog Nook and Cranny and determined I would make them as soon as possible. That was several months ago. Timely I'm not. Determined I am. I've made a single change to the base recipe which I'm told first appeared on All Recipes. You'll see that I've substituted some brown sugar for a portion of the white in an effort to make the bars more flavorful. I think it worked. If you use frozen berries do not thaw before using. Here's the recipe.

Blueberry Crumb Bars

Ingredients:
3 cups all-purpose flour
1/2 cup light brown sugar
1/2 cup + 1/4 cup granulated sugar, divided use
1 teaspoon baking powder
1 cup cold butter (2 sticks or 8 ounces)
1 egg, beaten
1 teaspoon of vanilla extract
1/4 teaspoon salt
Zest and juice of one lemon
4 cups blueberries
4 teaspoons cornstarch
Optional: Confectioners' sugar for dusting

Directions:
1) Preheat the oven to 375 degrees F. Butter and flour a 9 x 13 baking pan.
2) In a medium bowl whisk together 3 cups of flour, 1/2 cup brown sugar, 1/2 cup granulated sugar, baking powder, salt, and lemon zest. Use a pastry cutter or fork to cut in butter until mixture is in pieces no larger than a pea. Mix in egg and vanilla extract. Dough will be crumbly but stick together when squeezed. Pat half of the dough into the pan.
3) In another bowl, mix reserved 1/4 cup granulated sugar, cornstarch, and lemon juice. Add blueberries and gently toss them to coat. Pour blueberry mixture on top of the dough in pan. Then crumble remainder of dough evenly over surface of berries.
4) Bake in the preheated oven for 45 to 60 minutes, or until the top is slightly browned. Cool completely, then cut into squares. Dust with powdered sugar if using. Can be refrigerated for several days. Yield: 15 bars.