Preheat oven to 450°F. Spray a 12-inch cast-iron skillet or a large round baking pan with baking spray.

Grate the butter using the fine side of a grater onto a piece of parchment paper, then place it in the freezer for 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 and reserve the rest for the tops.

In a large bowl, whisk together the flour, granulated sugar, baking soda, and baking powder.

Incorporate the grated butter into the dry ingredients using a fork.

Pour in the buttermilk and stir until mostly combined. Gently fold the dough over a few times until sticky and no dry bits remain. If too dry, add 1-2 tablespoons of buttermilk.

Add the blueberries and fold a few more times until just incorporated. If using frozen blueberries, do not thaw.

Divide the dough into 7 equal portions and 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.

Brush the tops with heavy cream and sprinkle with the remaining cinnamon sugar mixture.

Bake for 22-27 minutes, or until the tops are golden brown and the biscuits are puffed up.

Remove from the oven and brush the tops with melted butter.

Serve warm.