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Preheat the oven to 425 degrees F. Grease a cast iron skillet or line a medium baking sheet with parchment and set off to the side.
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In a large mixing bowl, whisk together the flour, sugar, baking powder, cream of tartar, and salt.
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Add the cold cubes to the flour. Use a pastry cutter , two knives, or a fork to work the butter into the flour. The butter will end up the size of small pebbles and oat flakes when done. Mix in the buttermilk, cranberries, and cheddar until a shaggy dough forms.
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Turn the dough out on a lightly floured surface. Gently form the dough into a 1 inch thick disc. Use a 2 and a half inch circle biscuit cutter to cut as many rounds as you can. Reform the dough scraps and cut out as many biscuits as you can.
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Lightly brush the tops of the biscuits with buttermilk, top with salt, and bake for 12-15 minutes until risen and lightly golden.
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Let cool for a few minutes before serving.