Preheat oven to 425. Trim scallions and chop.
Combine flour, baking powder, sugar and salt in a bowl. Using a box grater, grate cold butter and add to the dry mix. Using your fingers, mix the butter into the dry mix. Add scallions, sour cream and milk. Using a fork and working in circles, mix until large shaggy clumps form. Be careful not to overmix.
Turn out onto a lightly floured surface and knead once or twice until it comes together; flour your hands if needed. If there are any loose bits, add them to center of dough and knead once more to incorporate.
Form the dough into a square and then cut the dough into four squares and stack on top of each other. Press down and then flatten dough until it is about 1 inch thick. Using a biscuit cutter, cut out biscuits.
Transfer to a parchment paper lined baking sheet. Melt butter and brush overtop, followed by flaky sea salt. Bake for 12-15 min or until biscuits are golden brown.