In a medium bowl, whisk together the all-purpose flour, baking soda, salt, and ground cinnamon. In a larger bowl, mix melted butter with granulated sugar, brown sugar, lemon zest, and vanilla until smooth. Incorporate dry ingredients, set aside ¾ cup of dough for topping, and press remaining dough into a baking pan to form the bottom crust.

Preheat your oven to 350°F (175°C). Bake the crust for 14-16 minutes, until set and lightly golden. Allow cooling on a wire rack for about 15 minutes.

In a saucepan, combine blackberries, granulated sugar, a pinch of salt, cornstarch, lemon zest, and lemon juice. Cook over medium heat, stirring gently until boiling and thickening for 3-4 minutes. Simmer for 1-2 more minutes.

Pour the filling over the cooled crust and spread evenly. Sprinkle reserved crumb topping over filling, leaving gaps for filling to peek through. Bake for approximately 40 minutes, or until topping is golden brown and filling is bubbling.

Allow bars to cool completely in the pan on a wire rack. For clean slices, place in the freezer for about 15 minutes before slicing into squares.