Step 1

Preheat oven to 350F. Line an 8-inch square pan with aluminum foil and spray with cooking spray; set aside. Lining the pan will really help with cleanup and is recommended.

Step 2

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

Step 3

Add the sugars and whisk to combine.

Step 4

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

Step 5

Set a heaping 3/4 cup crumble mixture aside.

Step 6

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.

Step 7

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

Step 8

Add the flour and whisk to combine.

Step 9

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

Step 10

In a separate medium mixing bowl, add all filling ingredients and toss to combine. If sugar hasn’t dissolved fully or still has crystals, that’s okay because it liquifies while baking.

Step 11

Evenly distribute blueberry mixture over the filling.

Step 12

Evenly sprinkle with the reserved heaping 3/4 cup crust mixture. Before sprinkling, I squeeze the mixture in my palm to encourage bigger crumble pieces to form.

Step 13

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.*

Step 14

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. Bars will keep airtight at room temperature for up to 1 week, in the fridge for up to 10 days (they’re good slightly chilled if you like chilled pie), or in the freezer for up to 6 months.