Prepare the grill for indirect cooking over medium heat (350° to 400°F).
Prepare the grill for cooking over medium heat (400°F).
Grease a (10 inches) cast iron skillet with the 1 tablespoon butter.
Whisk the flour, cornmeal, baking powder, salt, and cinnamon in a medium bowl.
Lightly whisk the sour cream, vanilla, and lemon zest in a small bowl.
In the bowl of an electric mixer, cream the sugar and the ½ cup butter until light and fluffy, 3 to 4 minutes. Add the eggs, one at a time, mixing well after each addition. Add the flour mixture in thirds, alternating with the sour cream, and ending with the flour. Gently fold in the blackberries and the ¾ cup blueberries.
Spread the batter in the prepared skillet. Top with the remaining ÂĽ cup blueberries, gently pressing the berries to partially submerge in the batter.
Grill over indirect medium heat, with the lid closed, until a toothpick inserted in center comes out clean, about 40 minutes, rotating the skillet once or twice to ensure even cooking. Transfer the skillet to a rack to cool.
Grill over medium heat, with the lid closed, until a toothpick inserted in center comes out clean, about 40 minutes, rotating the skillet once or twice to ensure even cooking. Transfer the skillet to a rack to cool.
Cut into wedges and serve garnished with lemon zest and a dollop of whipped cream, if desired.