Preheat the oven to 350 degrees, and line an 8-inch square pan with parchment paper. Lightly spray with cooking spray. Set aside.
In a large bowl, beat together the butter and powdered sugar. Add in the salt and vanilla extract. Add in the flour and beat until a dough forms.
Press about 2/3 of the dough into the bottom of your prepared pan, spreading evenly. Bake in your preheated oven for 10-12 minutes, or until just slightly golden on the edges.
With the remaining dough, mix in the oats. This should create a more crumbly dough. Set aside, uncovered (to create a more dry crumb).
In a medium saucepan, add the blueberries, sugar, lemon zest, salt, and cornstarch. The mixture will seem dry, but the berries will release their juice as they cook. Stir over medium heat for about 10 minutes, until the mixture is boiling and thickened.
Spread the blueberry filling over the cooked crust. Sprinkle the crumb topping evenly over the top. Return to the oven for 20-25 minutes, or until the top becomes lightly golden brown. Cool before removing from the pan and slicing.