Toss strawberries, blueberries, sugar, Grand Marnier (if using), orange zest and cinnamon together. Let sit for at least 20 minutes or up to a day.
Stir together flour, sugar, baking powder and salt. Add the grated butter and stir just until the butter is incorporated.
Add buttermilk to the flour mixture and then turn out onto the counter and form the dough into a rectangle.
Fold the dough on the left and right sides. Flatten the dough, rotate and repeat three times to build flaky layers.
Cut out the shortcakes with a cookie cutter, brush with more buttermilk and bake in a hot oven.