Step 1

Preheat oven to 350°F and line an 8-inch square baking pan with parchment paper. Whisk together melted butter, sugar, brown sugar, vanilla, and lemon zest until combined.

Step 2

Whisk together flour, baking soda, salt, and cinnamon in a medium bowl; add to butter mixture and fold in until incorporated. Reserve 3/4 cup of dough for topping (keep chilled); press remaining dough evenly into bottom of prepared pan. Bake crust for 14 to 16 minutes, until lightly golden and set. Transfer to a wire rack and cool for 15 minutes.

Step 3

For the filling, combine sugar, salt, and arrowroot starch in a medium bowl. Add blackberries, lemon zest, and juice, tossing gently to combine. Spoon mixture over cooled crust and crumble reserved dough over filling.

Step 4

Bake for another 40 to 44 minutes, or until topping is golden and filling slightly bubbly. Cool completely in pan on a wire rack. Just before cutting, chill in the freezer for 10 minutes. Carefully lift parchment and place bars on a cutting board; cut into squares with a sharp knife.