1. Preheat oven to 400 degrees. Line 3 rimmed baking sheets with parchment. Whisk flour, baking powder, baking soda, salt, and granulated sugar in a bowl. Cut in butter with a pastry cutter until mixture resembles coarse crumbs. Add buttermilk, zest, and chopped tangelos, stirring just until dough comes together (some butter should be visible).

2. Turn dough out onto a floured surface, and knead a few times to bring together. Gently pat it into a 6-by-15-inch rectangle, 1 inch thick. Using a paring knife, cut dough into 20 triangles (3 inches each). Flip each triangle over onto a baking sheet, spacing each 2 inches apart. Cover, and refrigerate until cold and firm, at least 2 hours (or overnight). Brush tops with cream, then sprinkle with sanding sugar.

3. Bake scones until tops are golden brown, 22 to 25 minutes. (If bottoms are browning too quickly, place another baking sheet underneath the first.) Transfer scones to a wire rack, and let cool slightly. Serve warm.