Preheat a 350 degree oven. Line an 8 inch square baking pan with parchment paper.

Whisk together butter, brown sugar, and lemon zest until well incorporated in a large bowl.

Add flour, baking soda, salt, and cinnamon to the butter mixture until well incorporated.

Take one cup of crumb mixture and set aside for later.

Press the rest of the dough mixture into the bottom of the prepared baking dish and bake for 12-15 minutes. Let the crust cool completely.

In a medium mixing bowl combine flour, sugar, blackberries, lemon zest, and lemon juice until well incorporated.

Pour blackberry mixture on the cooled crust, and spread fruit mixture until even across the crust. 

Sprinkle the remaining crumble mixture that was set aside.

Bake the blackberry crumb bars for 45 minutes or until golden brown in the preheated oven.

Cool the bars for at least 30 minutes before cutting into individual bars.

You can serve them warm or chilled, depending on your preference. The bars are delicious on their own, so you can simply serve them as is, but if you're feeling extra indulgent, top with a scoop of vanilla ice cream or whipped cream!