Preheat oven to 375 degrees.
Line 8x8 baking dish with parchment & spritz lightly with cooking spray.
In a small bowl- combine 1/4 cup sugar & cornstarch - stir in lime juice.
Add blackberries - toss to coat - set aside.
In a larger bowl- combine flour, 1/2 cup sugar, baking powder & salt.
Add egg yolk, vanilla & butter cubes.
Using a pastry tool- work the mixture together until it resembles coarse crumbs.
Spread 2/3 of the mixture into prepared pan - pressing down to even out & flatten.
Top with blackberry mixture - spreading them into an even layer.
Sprinkle remaining crumb mixture over top & then sprinkle with remaining sugar.
Bake for 35-40 minutes or until toothpick test is clean.
Let cool completely before cutting & serving- unless you are serving with ice cream, which is better with a warm bar.
Cool about 15 minutes in pan before cutting when serving with ice cream.