1Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
2Mix flour, brown sugar, white sugar, butter, and salt together in a bowl until crumbly. Measure 1 cup mixture and set aside for later use.
3Stir buttermilk, eggs, and baking soda into crumbly mixture until batter is smooth. Pour batter into the prepared baking dish. Sprinkle reserved crumbly mixture over top of batter; top with cinnamon.
4Bake in the preheated oven until a toothpick inserted in the middle comes out clean, 45 to 50 minutes.