Preheat the oven to 425°F, and line a baking sheet with parchment paper.
Reserve 1 tablespoon of the sugar, and set aside for now. Add the flour, sugar, baking powder and soda, cinnamon, salt and orange zest in a bowl. Stir to blend.
Cut the cold butter into the mixture using two knives or a pastry blender. The mixture should be coarse, with crumb-sized grains.
Reserve 1-2 tablespoons of the buttermilk. In a separate bowl, beat the rest of the buttermilk, egg yolk, and orange juice together.
Stir the buttermilk mixture into the flour mixture. Fold it over only until combined – do not over mix.
Transfer the dough onto a lightly floured surface, and knead it gently for no more than a few seconds.
Form the dough into a disk just under an inch thick. Cut it into 3-inch biscuit rounds. You can use a biscuit cutter or a glass with the right dimensions – just be sure to flour it lightly first so it doesn’t stick.
Transfer the dough rounds to the baking sheet. Brush with the reserved buttermilk, and sprinkle with reserved sugar.
Bake for 12 to 15 minutes, or until lightly golden brown.
Allow the shortcakes to cool on a rack for about 10 minutes, and then, using a serrated knife, cut them in half.