Preheat your oven to 400°F. Mix sugar, flour, baking soda, baking powder, and salt. Cream or pulse the mixture until everything is combined.
Toss in your cubed butter. Pulse it a few times until you get coarse crumbs.
Pour the buttermilk in and gently fold everything in.
Move the dough onto a floured surface. Use your floured hands to bring it together.
Flatten the dough into a thick rectangle (roughly ¾ inch). Fold one of its sides into the middle, then the remaining side. Turn that dough horizontally and gently flatten it into a rectangle again. Redo 2-3 more times.
Use your biscuit cutter to cut out circles (about 3 inches wide) from the dough. If some scraps are left, re-roll or flatten them.
Arrange your biscuits in a 10-inch iron skillet or line a baking sheet with silicone baking mats or parchment paper. Make sure the biscuits touch.
Brush their tops with buttermilk or heavy cream and sprinkle coarse sugar. Bake for about 20 minutes (or 'til the tops turn golden brown).
Let them cool in their pan for 10 minutes or longer.