Prepare two baking sheets with parchment paper.
In a large bowl, whisk flour, scallions, chives, black pepper, baking powder, salt, and baking soda. Add cheese and grated butter and toss until fully coated. Make a well in the center of the flour mixture and add buttermilk. Mix until a shaggy dough forms and turn out dough onto a floured surface.
Shape dough into a 9 inch by 9 inch square and lightly dust with flour. Fold into thirds like a letter. Dust again with flour and rotate 90°. Complete the process of folding into thirds and rotating two more times. Shape back into a 9 inch by 9 inch square. Cut into 16 (or 9, for bigger biscuits) equal squares and transfer to a prepared sheet pan. Refrigerate 30 minutes.
Preheat oven to 400°F. Arrange biscuits on prepared sheet pans so that there are 8 on each pan, each at least 2 inches apart. Brush with melted butter and top with flaky salt and freshly cracked black pepper. Bake until golden brown, rotating pans halfway through, 25-30 minutes.