Spread out the garbanzos on a baking sheet and let them dry, uncovered, in the refrigerator overnight. (If you forget to let them dry overnight, it's okay; you'll just need to cook them longer.)
Place the bacon in a large skillet over medium-low heat and cook until browned and crispy, 4 to 5 minutes per side. Drain on a paper-towel-lined plate; reserve the fat in the pan. Eat the bacon as snack while you cook.
Preheat the oven to 400 degrees F.
Add the dried garbanzos to the pan with the rendered bacon fat. Season with a generous pinch of salt and add the rosemary. Cook over medium-low heat for about 2 minutes, then remove the rosemary sprig. Transfer the garbanzos to a baking sheet and bake until crispy, 35 to 40 minutes, stirring every 10 minutes. (If you did not dry the garbanzos overnight, bake them for 45 to 50 minutes.)
Let cool to room temp before serving.