1. Preheat oven to 350° F. Line an 8-inch square pan with aluminum foil (will help with cleanup) and spray with cooking spray; set aside.

For the Crust & Crumble Topping

1. In a large, microwave-safe bowl melt the butter, about 1 minute on high power.
2. Add the sugars and whisk to combine.
3. Add the flour and optional salt, and stir to combine. Mixture will be very dry and sandy with some larger, well-formed pieces.
4. Set a heaping ¾ cup crumble mixture aside.
5. 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.

For the Filling

1. In a medium bowl, add egg, Greek yogurt, sugar, vanilla, and whisk to combine.
2. Add the flour and whisk to combine.
3. Evenly pour filling over the crust and jiggle pan to help distribute it, or smooth it with a spatula; set aside.

For the Blueberry Layer

1. In a separate medium mixing bowl, add all filling ingredients and toss to combine. If sugar hasn’t dissolved fully or still has crystals, the sugar will liquify while baking.
2. Evenly distribute the blueberry mixture over the filling.
3. Evenly sprinkle with the reserved heaping ¾ cup crust mixture. Before sprinkling, you can squeeze the mixture in your palm to produce bigger crumble pieces.
4. Bake for about 60 minutes, or until edges are bubbling quite vigorously and there is some bubbling in center. Crumble topping should appear set and very pale golden.
5. Place pan on a wire rack and allow bars to cool for at least 1 to 2 hours (better to cool overnight) before slicing and serving.