Instructions

Adjust oven racks to upper-middle and lower-middle positions and heat oven to 200 degrees. Maintain temperature for 10 minutes, then turn off oven. Line two baking sheets with parchment paper.
Stir buttermilk and yeast together until dissolved. In bowl of standing mixer fitted with paddle attachment, mix flour, baking powder, baking soda, sugar, and salt on low speed until combined. Add shortening and mix until just incorporated, about 1 minute. Slowly mix in buttermilk mixture until dough comes together, about 30 seconds. Fit mixer with dough hook and mix on low speed until dough is shiny and smooth, about 2 minutes.
On lightly floured surface, knead dough briefly to form smooth ball. Roll dough into 10-inch circle, about 1/2 inch thick. Using 21/2-inch biscuit cutter dipped in flour, cut out rounds and transfer to prepared baking sheets. Gather remaining dough and pat into 1/2-inch-thick circle. Cut remaining biscuits and transfer to baking sheets. Shape according to photos at right. Cover dough with kitchen towels and place in warm oven. Let rise until doubled in size, about 30 minutes.
Remove baking sheets from oven and heat oven to 350 degrees. Once oven is fully heated, remove kitchen towels and bake until biscuits are golden brown, 12 to 14 minutes, switching and rotating sheets halfway through baking. Remove from oven and brush tops with melted butter. Serve.
Food Processor Method: In step 2, process flour, baking powder, baking soda, sugar, and salt until thoroughly mixed. Add shortening and pulse until only pea-sized pieces of shortening remain, about 6 one-second pulses. Add buttermilk mixture in steady stream and pulse until dough comes together, about 6 more pulses. (Dough will be very sticky.) Turn dough out onto floured work surface and knead until shiny and smooth, about 5 minutes, adding more flour as necessary. Continue recipe with rolling out dough as directed in step 3.