Preheat oven to 450°F. Spray a 12-inch cast-iron skillet or a large round baking pan with baking spray.
Using the fine side of your grater, grate the butter onto a piece of parchment paper. Place the paper with the butter in the freezer while you prepare the remaining ingredients, about 5 minutes.
In a small bowl, mix the brown sugar and cinnamon.
Sprinkle 1 tablespoon of the cinnamon sugar mixture evenly into the bottom of the prepared pan. Set aside. Reserve the rest of the cinnamon mixture for the tops.
In a large bowl, whisk the flour, sugar, baking soda, and baking powder.
Using a fork, incorporate the grated butter into the dry ingredients.
Pour in the buttermilk and stir until mostly combined. Use your hands to gently fold the dough over itself a few times, mixing until the dough is sticky and no dry bits remain. If the dough is too dry, add 1-2 tablespoons of buttermilk.
Add the blueberries and gently fold the dough a few more times until they are just incorporated. If using frozen blueberries, do not thaw.
Divide the dough into 7 equal portions (about 1 cup or 7 ounces each) and gently shape each into a rough ball.
Arrange 6 balls of dough around the edge of the pan and place 1 in the center, touching but not stacked. (They should be large.)
Brush the tops with heavy cream, then sprinkle evenly with the remaining cinnamon sugar mixture.
Bake for 22-27 minutes, or until the tops are golden brown, the biscuits are puffed up, and the centers between biscuits no longer look wet or doughy.
Remove the biscuits from the oven and immediately brush the tops with melted butter.
Serve warm.