Prep butter & citrus: Grate or dice frozen butter into pea-sized pieces; keep in freezer. Zest & juice oranges, dividing between dough & glaze.
Mix dry ingredients: In a large bowl, combine flour, cornstarch, sugar, baking powder, salt, half the zest. Cut in butter until pea-sized pieces remain.
Form dough: Stir in starter, cream, cranberries, ¼ cup orange juice until just combined. Do not overmix. Shape into ball, wrap, refrigerate 30 min–48 hrs.
Shape scones: Preheat oven 400°F. Flatten dough to 1” thick circle; cut into 8 wedges (or use biscuit cutter). Place on parchment-lined sheet, 2” apart; freeze for 5–10 min before baking.
Bake: 15–20 min, until golden.
Glaze: Mix melted butter, powdered sugar, remaining zest, orange juice to pourable consistency. Drizzle over cooled scones. Optional: add cranberries or cranberry juice for variation.