Preheat oven to 375F. Line a 9x13-inch pan with parchment paper leaving an overhang on opposite sides to lift the bars out after baking.

In a large bowl, combine the sugar and lemon zest. Rub the zest into the sugar until the sugar is moist and fragrant. Add the flour, baking powder, and salt to the bowl and whisk to combine. Add the butter and egg to the bowl, and use a pastry cutter to cut them in until the pieces of butter are no larger than small peas – the mixture should look like coarse meal. Transfer half of the mixture to the prepared pan and pat into an even layer to form the crust.

In a medium bowl, stir together the lemon juice, sugar, and cornstarch. Add the blueberries, and toss gently to coat them evenly. Pour the blueberry mixture over the crust and spread evenly. Crumble the remaining crust/topping mixture over the blueberries to cover.

Bake for 40-45 minutes, or until topping is golden brown and the filling bubbles around the edges. Remove the pan to a wire rack and allow the bars to cool completely in the pan. Once cool, use the parchment “handles” to lift the bars out and cut into squares (I actually like to refrigerate them for an hour or two before cutting – it makes for neater squares). Store in an airtight container in the refrigerator.