Make the black bottom
Line 12 regular-sized (2-3/4-inch diameter) muffin cups with foil liners (paper ones won’t work here) and lightly grease the bottom and sides.
Put the cookie crumbs and butter in a small bowl and mix with a fork until the crumbs are evenly moistened.
Portion the crumbs evenly among the foil liners (1 firmly packed tablespoon in each). If you have a tart tamper, this is a great time to use it. If not, tear off a piece of plastic wrap, place it over the crumbs, and use your fingers to press firmly to make a compact layer. Slide the tin into the refrigerator while you make the filling. (This helps keep the crumbs on the bottom when you are swirling the batters together.)
Make the cheesecake
Position an oven rack in the center of the oven and heat the oven to 300°F. Put the cream cheese in a large bowl and beat with an electric mixer fitted with the paddle attachment on medium high until very smooth and fluffy, about 2 minutes. Add the sugar, vanilla, and salt. Continue beating until well blended and smooth, scraping down the sides of the bowl frequently; there should be no lumps. Add the eggs, one at a time, beating on medium speed until just blended. Once the eggs have been added to the batter, it’s important to mix thoroughly without overbeating or the cheesecake will puff and crack during baking.
Scoop out 1 cup of the vanilla batter and set aside. Add the Nutella to the remaining batter and stir until well blended. Portion the Nutella batter evenly among the prepared muffin cups (about 4 Tbs. per cup). Next, portion the vanilla batter evenly among the cups (about 1 Tbs. in each). The cups should be completely filled.
Drag and fold the tip of a wooden skewer, toothpick, or paring knife through the two batters in a swirled pattern. There should still be solid streaks of each batter visible, and the cups should be filled to the brim. Tap the muffin tin on the counter to settle the batters. Bake until the centers barely jiggle when the pan is nudged, 20 to 22 minutes. Set the muffin tin on a rack and let cool completely. Cover and refrigerate until very cold, at least 6 hours.
The baked cheesecakes can be refrigerated for up to 3 days or frozen for up to 1 month. Once cooled, cover the muffin tins with plastic (tin and all to keep the foil perky and pretty), then refrigerate or freeze.