Prepare the oven and pan. Heat the oven to 425°F (220°C). Line a baking sheet with parchment rated for that temperature.

Combine the dry ingredients. Whisk the flour, sugar, baking powder, salt and cream of tartar in a large bowl.

Work in the cold butter. Add the butter cubes. Use a pastry blender or your fingertips to break them into smaller pieces, leaving some visible flakes. Stop if the mixture starts feeling greasy; chill it before continuing.

Add the buttermilk. Pour in the buttermilk and stir gently with a fork until a rough, shaggy dough forms. If loose flour remains after gentle mixing, add extra buttermilk a tablespoon at a time, just until the dough holds together.

Fold and shape. Turn the dough onto a lightly floured surface. Press it into a rectangle, fold it in thirds, then gently flatten it again. Repeat once. Pat to approximately 3/4–1 inch thick.

Cut the biscuits. Use a roughly 2 1/2-inch cutter, pressing straight down. Gather scraps gently for another cut; avoid repeatedly kneading them. Set the biscuits on the prepared sheet with a little space between them.

Chill briefly. Refrigerate the shaped biscuits for about 10 minutes, especially if the dough has warmed during shaping.

Bake and finish. Bake for approximately 15–20 minutes, checking for golden tops, lightly browned bases and a center that is cooked through rather than wet or gummy. Brush with the melted butter and serve warm.