I never have buttermilk on hand when I want homemade flaky Southern biscuits. This one ingredient makes a perfect substitute.
In a medium bowl, combine the first 5 ingredients. Add in the butter and shortening and mix together with your hands, until it is incorporated.
In a small bowl, whisk together the Greek yogurt and milk. Add it to the other bowl, and combine with your hands.
Dump the dough onto the counter. Do not knead it. Press the dough together into disc about 1-inch thick. Using a glass or two-inch biscuit cutter, cut out 6-8 circles. You can press the edge pieces together to reform the disc, as needed. Place the biscuits onto a lined baking sheet. Refrigerate for 30 minutes.
Heat the oven to 375 degrees. Brush the biscuits with melted butter and a sprinkle of salt and bake for 12-15 minutes, until golden on top.