Prepare the grill for indirect cooking over medium heat (350°F).
Place a large foil pan on top of the Flavorizer bars, below the cooking grate. Fill it halfway with room temperature water. Prepare the grill for cooking over medium heat (350°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 ¼ 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 above foil pan, 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.
