Blueberry Crumble Bars

Preheat oven to 375 degrees F and grease a 9x13 inch baking pan (I also lined my pan with parchment paper for easy transfer and cutting).
In a large bowl, mix together the 3 cups of flour, 1 cup of sugar and baking powder. Mix in salt and lemon zest. Using a large fork or pastry cutter, blend the butter and egg into the flour (you could also do this is a food processor, but be careful to slowly pulse until just incorporated). The dough will be very crumbly. Transfer half the dough to your prepared pan and pat firmly until crust is uniform.
In a separate bowl, stir together the 1/2 cup sugar, cornstarch, and lemon juice. Mix in the blueberries. Transfer the blueberry mixture over the top of the prepared crust. Finally, crumble remaining dough over the top of the berry layer.
Bake in preheated oven for approximately 45 minutes, or until top is slightly brown. Remove from the oven and allow bars to cool completely before cutting into squares, or, better yet, store in the refrigerator for a few hours before serving (it makes slicing a breeze and, in my opinion, they are best cold).