Preheat your oven to 350°F and prepare an 8-inch square baking pan by lining it with parchment paper.

In a medium bowl, combine the all-purpose flour, baking soda, salt, and cinnamon. Whisk until well mixed.

In a large bowl, whisk together the melted unsalted butter, granulated sugar, brown sugar, vanilla extract, and lemon zest until fully combined.

Fold the dry flour mixture into the butter mixture until just combined, creating a crumbly dough.

Set aside 3/4 cup of the dough for the crumb topping, and press the remaining dough evenly into the bottom of the lined baking pan.

Bake the crust in the preheated oven for 14-16 minutes until lightly golden, then cool for about 15 minutes.

In a saucepan over medium-high heat, combine the blackberries, granulated sugar, salt, cornstarch, lemon zest, and lemon juice. Cook until bubbling for about 3-4 minutes.

Once boiling, reduce the heat and simmer for an additional 1-2 minutes until the filling thickens slightly. Cool for about 10 minutes.

Spoon the blackberry filling evenly over the cooled crust, then crumble the reserved dough on top.

Return the pan to the oven and bake for another 40 minutes until the topping is golden-brown.

Cool completely in the pan before slicing.