Preheat oven to 350F. Optionally, line an 8-inch square pan with nonstick aluminum foil and spray with cooking spray; set aside. If you have other foil, or don't want to use it because you fear it will stick to the dessert, then omit the foil and just spray your pan well with cooking spray.

In a large, microwave-safe bowl melt the butter, about 1 minute on high power.

Add the sugars (0.5 cup granulated, 0.25 cup light brown) and whisk to combine.

Add the 1.5 cups flour, optional salt, and stir to combine. Mixture will be very dry and sandy with some larger, well-formed crumble pieces.

Set a heaping 0.75 cup crumble mixture aside.

Transfer remaining mixture to prepared pan, and using a spatula or your fingers, hard-pack the mixture to create an even, smooth, flat crust; set aside. Press it down but do not hard-pack it or the crust will bake up too hard to cut easily.

In a medium bowl, add the egg, Greek yogurt, 0.33 cup sugar, vanilla, and whisk to combine.

Add the 1 tablespoon flour and whisk to combine.

Evenly pour filling over the crust and jiggle pan to help distribute it or smooth it with a spatula; set aside.

In a separate medium mixing bowl, add all blueberry filling ingredients (blueberries, 0.33 cup sugar, lemon juice, cornstarch) and toss to combine. If sugar hasn’t dissolved fully or still has crystals, that’s okay because it liquifies while baking.

Evenly distribute blueberry mixture over the filling.

Evenly sprinkle with the reserved heaping 0.75 cup crust mixture. Before sprinkling, squeeze the mixture in your palm to encourage bigger crumble pieces to form.

Bake for about 60 minutes, or until edges are bubbling quite vigorously and there’s some bubbling in the center. Crumble topping should appear set and very pale golden.

Place pan on a wire rack and allow bars to cool for at least 1 to 2 hours (overnight is even better) before slicing and serving. If you try to slice bars before they’ve cooled completely, they’ll be extremely messy and could fall apart.