Berries: Add half of the blackberries to a saucepan with 2 Tablespoons sugar and 1 Tablespoon lemon juice. Stir. Cook for a few minutes over medium heat, breaking the blackberries in half with your spatula or wooden spoon, until they've cooked down a little bit and formed a little juicy sauce.
Prep pan: Preheat oven to 350°F (180°C). Slice butter into pieces and add to a 9x13 inch baking dish. Place the pan in the oven while it preheats, to allow the butter to melt. Once melted, remove the pan from the oven. (You could also bake this is a dutch oven or 12'' cast iron skillet)
Batter: In a large bowl mix together the flour, sugar, baking powder, and salt. Stir in the milk, just until combined. Pour the mixture into the pan, over the melted butter and smooth it into an even layer.
Assemble: Spoon the blackberry sauce mixture over the top, and fill in gaps with the remaining whole blackberries. Sprinkle with a little ground cinnamon.
Bake at 350 degrees for about 35-40 minutes or until golden and slightly crisp on top. Serve warm, with a scoop of ice cream, if desired.
