Preheat the oven to 450 degrees F. Line a large rimmed baking sheet with parchment paper. Smash the garlic cloves and peel off the skin. Then pour the bacon grease or oil on the baking sheet and lay the garlic cloves, and rosemary on the sheet pan as well.
Hold the end of one potato. Use a sharp knife to cut across the potato to create 1/16- to 1/8-inch sections. Do not cut through the potato, but cut two-thirds the way down, leaving at least a 1/2 inch solid base at the bottom. Repeat with the remaining potatoes.
Place the potatoes on the baking sheet and roll them around to cover them with fat. Set them cut-side-up. Then sprinkle each potato generously with salt and pepper.
Roast in the oven for 30 minutes. Then use tongs to gently squeeze the base of each potato to open up the folds. Use a pastry brush to brush the herb-infused pan oils over the top of each potato and in between the folds. Discard the garlic cloves if they are blackened.
Roast in the oven for another 30 minutes, until the largest potato is fork tender. Serve warm.
You can use potatoes other than Yukon Gold, such as red potatoes or white potatoes; however, I find that you don't get quite the same incredibly crispy texture as yellow Yukon potatoes.