1 Preheat the oven and prep the pan: Preheat oven to 375°F. Butter and line an 8×8 inch pan with parchment paper leaving an overhang on two sides.
2 Make the dough: In a food processor, pulse the flour, sugar, lemon zest, and salt together a few times to combine. Add the butter and pulse until you get a fine, sandy texture—all of the butter should be incorporated.
Add the egg yolk and pulse until it’s combined, 10 to 15 times. The crumbs should hold together when squeezed in the palm of your hand. If they do not, sprinkle a tablespoon of water over top and pulse a few more times.
If you don’t have a food processor, you can cut the butter into the flour with a pastry blender or a fork. It will take longer, but work just fine.
3 Refrigerate the crumb topping: Scoop out about 1/3 of the crumb mixture and press it into a bowl, which helps to make larger clumps of crumbs for the topping. Cover and refrigerate for at least 15 minutes or until needed for the topping.
4 Bake the bottom crust: Firmly press the remaining 2/3 of the crumbs into the bottom of the prepared pan to create the base for the bars. I like to use a measuring cup to press the crumbs into the pan and really compact the crust.
Bake for 15 minutes. Check the crust partway through cooking, and if it’s puffed up, press it down gently with a spatula. Tip: This step is called par-baking, because you are partially baking the crust.
5 Make the blueberry filling and top with crumbs: In a mixing bowl, toss the blueberries with the sugar, cornstarch, lemon juice, and zest together to coat. Spread the berries over the par-baked crust.
Remove the remaining crumb mixture from the fridge and crumble. Crumble it with your fingers and sprinkled over the blueberry filling.
6 Bake the crumb bars: Bake for 45 minutes, or until the blueberries are bubbling and the crumb topping is golden. Remove the bars from the oven and transfer to a cooling rack. Cool completely in the pan before cutting, at least 2 hours.
Once cooled and sliced, the bars can be transferred to an airtight container and kept at room temperature for about a week.