Ms. Malkin’s operation, Fort Mill Sourdough, is one of a remarkable number of microbakeries proliferating across the United States, in big cities and small towns alike. Sky-high commercial rents and the impossible math of how to care for children while running a business have created the need for home-based bakeries. Flexible state licensing, digital tools and a hunger for sourdough — which many consumers believe is healthier than mass-produced bread — have created big opportunities for home bakers.