Prep the pan and oven. Heat the oven to 350°F (175°C).
Line a standard 12-cup muffin tin with paper liners. Lightly mist the liners with nonstick spray for easy release.
Make the cream cheese filling. In a medium bowl, beat the softened cream cheese, sugar, egg, vanilla, and salt until smooth and creamy. Fold in the mini chocolate chips.
Set aside.
Whisk the dry ingredients. In a large bowl, whisk flour, sugar, cocoa, baking soda, and salt (and espresso powder if using) until evenly combined with no cocoa clumps.
Add the wet ingredients. Make a well in the dry mixture. Pour in water, oil, vinegar, and vanilla. Whisk just until the batter is smooth. Do not overmix; a few small lumps are fine.
Portion the chocolate batter. Fill each cupcake liner about half full with the chocolate batter.
You should use roughly 2 tablespoons per cup, leaving room for the filling.
Add the cream cheese centers. Spoon about 1 heaping tablespoon of the cream cheese mixture onto the center of each cupcake. The filling will sink slightly as it bakes and create the classic “black bottom” effect.
Bake. Place the pan on the middle rack and bake for 20–24 minutes. The chocolate cake should spring back lightly when touched, and a toothpick inserted near the cake portion (not the creamy center) should come out with just a few moist crumbs.
Cool. Let the cupcakes cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
The centers will set as they cool.
Serve. Enjoy at room temperature for a soft, tender bite, or chill for a firmer, cheesecake-like center. Both ways are excellent.