Preheat the oven to 425℉ (218 ℃). Grease a cast-iron skillet or line a baking sheet.
Place the butter in the freezer for 15-20 min if it's not already super cold.
Measure the flour using the scoop-and-level method. (see note) Whisk flour, baking powder, sugar, and kosher salt together.
Use a box grater to grate the butter into the flour mixture and gently toss with your hands or a fork until the butter is coated.
Create a well in the flour mixture and add the buttermilk and the optional heavy cream. Stir gently just until the dough starts to come together. The dough should look shaggy and slightly messy. DO NOT overmix.
To create layers, turn the dough onto a floured surface and use your hands to press the dough into a rectangle.
Fold the dough in thirds. This is done by folding one side of the dough to the middle, and bringing the other side over that. Similar to folding a letter.
Rotate the dough 90°, gently press the rectangle out with your hands until thin enough to fold, and repeat the folding process. Repeat the folding a total of 3- 4 times. The dough should feel smooth.
For cutting, press the dough until it's 1- ½ inches thick. Cut the dough with a 3 inch biscuit cutter dusted with flour. Press straight down, do not twist. (You can also use a 2 ½ inch biscuit cutter instead)
Freeze shaped biscuits for 10-15 minutes before baking.
Arrange the biscuits in the greased cast iron pan or on a baking sheet. Place them close together but not touching. If you prefer a softer edge, have the biscuits touching.
Bake for 15-18 minutes at 425℉ (218 ℃) until the tops start to brown. Brush the tops with butter and serve warm.