In a large bowl, whisk together the flour, sugar, baking powder, and baking soda.
In a separate container, mix together the buttermilk, 5 tablespoons of the melted butter, and vanilla extract and pour over the flour mixture.
Using a wooden spoon, stir together until a shaggy mixture forms. Knead a few times with your hands until dough is smooth and divide into two equal portions.
Turn dough out onto a lightly floured surface, and press or roll out each piece into an 8x12-inch rectangle. Sprinkle the cinnamon filling evenly over each piece of dough, leaving a 1/4-inch border. Using your hands, gently pat the filling into the dough.
Add apple chunks on top of the cinnamon filling.
Starting with the longest side nearest you (12-inch edge), roll the dough into a log. Pinch the ends closed and cut each log into 8 equal rounds, for a total of 16 rolls.
Arrange the rolls in the prepared baking dish and brush tops with remaining butter. Bake until golden brown, about 20 minutes.
After removing from oven, let rolls cool in the pan on wire rack for 5 minutes.