In a deep-sided skillet over medium heat, melt 1 tablespoon of the butter.
Add the ground chicken to the melted butter.
Season the ground chicken with the red pepper flakes, the rubbed sage, 1/4 teaspoon of the salt, and 1/4 teaspoon of the black pepper.
Cook the chicken, while crumbling, until it is no longer pink, about 5-7 minutes.
Using a slotted spoon, transfer the ground chicken to a plate and reserve the drippings in the skillet.
Add 1 tablespoon of the remaining butter to the reserved drippings in the skillet and allow it to melt.
Add the onions to the butter mixture in the skillet and cook, while stirring, until they are translucent, about 2-4 minutes.
Add the carrots, the celery, the corn, the garlic, the extra salt, and the extra black pepper to the onions and cook, while stirring, until the carrots are slightly softened, about 5 minutes.
Add the ground chicken to the vegetable mixture in the skillet, stirring to combine.
Add 2 tablespoons of the remaining butter and 2 tablespoons of the flour to the chicken mixture in the skillet and stir to combine.
Cook the chicken mixture, while stirring, until the butter is fully melted, the flour fully coats the vegetables, and the raw flour smell disappears, about 2 minutes.
Add the chicken broth to the chicken mixture and stir to combine, while scraping the bottom of the skillet to release the browned bits and ensuring the flour dissolves entirely.
Bring the broth to a simmer and cook until it thickens slightly.
Taste the filling mixture for seasoning and add the extra salt and the extra black pepper, as needed.
Transfer the filling mixture to a 9x13-inch casserole dish.
Preheat the oven to 425 degrees F.
In a medium bowl, add the remaining flour, the baking powder, 1/2 teaspoon of the remaining salt, 1/4 teaspoon of the remaining black pepper, and the sugar and whisk to combine.
Add the cheese to the flour mixture and toss to combine.
In a small bowl, add 2 tablespoons of the remaining melted butter and the buttermilk and stir to combine. Don't worry if the mixture curdles.
Add the buttermilk mixture to the flour mixture and fold until just combined into a dough.
Drop dollops of the dough evenly over the top of the filling mixture in the casserole dish.
Cut the remaining 1 tablespoon of the chilled butter into very small pieces.
Sprinkle the butter pieces over the dough in the casserole dish.
Bake until the biscuits are golden-brown and the filling is slightly bubbly, about 25 minutes.
Serve garnished with the parsley.