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.