Place flour, butter, sugar, baking powder and salt together in a food processor. Pulse six times.
2 cups flour, ½ cup sugar, 1 teaspoon baking powder, ¼ teaspoon salt, ¾ cup butter
Add egg and pulse six more times. The mixture will be very dry
1 egg
Set one cup of crust aside to use for the topping
Place remaining crust mixture into an 8x8 or 9x9 baking dish that has been lightly sprayed with a nonstick spray.
Press crust down firmly with hands or the bottom of a flat measuring cup to form an even surface
Filling
Combine rhubarb, sugar, lemon zest, juice and cornstarch together
4 cups rhubarb, ½ cup sugar, 1 tablespoon lemon zest, 2 teaspoons cornstarch, 2 tablespoons lemon juice
Pour mixture over prepared, unbaked crust
Sprinkle on reserved crust mixture. Form larger chunks by squeezing the dough into your hand and then drop onto the rhubarb.
Bake in a preheated 375° oven for 45 minutes
Allow bars to cool before slicing
Notes
Baking Tips
Use cold butter that has been cut into small cubes
To incorporate the butter into the flour mixture, use a food processor, a pastry blender, or your hands. Smear the butter between your fingers mixing it into the flour. The mixture should be crumbly with no chunks of butter left.
Dice rhubarb into small cubes
If your rhubarb is stringy, just pull off any long strings that form
If using frozen rhubarb, do not thaw before using it. Increase baking time by 10 minutes.
Be certain to scrape any juices that were formed from the rhubarb sugar mixture into the baking dish
Allow the dessert to fully cool before slicing and serving