Preheat the oven to 375 degrees F. Add the 2 tablespoons of butter to an oven-safe or cast iron 10-inch skillet.
Separate the eggs. Place the whites in a bowl and beat them with a hand mixer until soft peaks form. Set aside.
Place the skillet with the butter in the oven to heat up.
In a large bowl, whisk together the flour, sugar, baking powder, soda, salt and cinnamon. In a smaller bowl, whisk together the buttermilk, egg yolks, vanilla extract and melted butter. Add the wet ingredients to the dry and mix until just combined. Fold in the egg whites until fully incorporated - folding and not mixing. Stir in the blackberries.
Remove the skillet from the oven. The butter should be melted. Pour the batter into the skillet. Bake for 25 to 30 minutes, or until golden brown.
Let cool slightly, then serve with maple syrup, whipped cream and powdered sugar.