Though unfrosted, these chocolate cupcakes with a marble-topped cream cheese filling aren't lacking in flavor or appearance. Their smooth tops also make them ideal candidates for baking cupcakes to-go.
Preheat oven to 350 degrees. Line two standard 12-cup muffin tins with liners.
Cupcakes: In a small bowl or liquid measuring cup, whisk together cocoa and hot water until smooth. In a separate bowl, whisk together flour, salt, baking soda, and baking powder. In a small saucepan, heat butter and sugar over medium-low, stirring occasionally, until butter melts and mixture is combined; transfer to a medium bowl and whisk until cooled slightly, 4 to 5 minutes. Add egg and whisk until smooth. Add cocoa mixture, flour mixture, sour cream, and vanilla; stir until just combined (do not over-mix).
Filling: In a medium bowl, whisk together cream cheese, sour cream, sugar, eggs, vanilla, and salt until smooth.
Spoon 2 to 3 tablespoons cupcake batter into each muffin cup, tapping tins firmly to level and evenly distribute batter. Evenly divide the cream cheese filling among the muffin cups, adding spoonfuls to each to fill muffin cups about three-quarters full. Dot remaining cupcake batter on top. Using a skewer or tip of a paring knife, make figure-8 motions to create a marbled effect on the surface of each cupcake. Bake until filling sets, 22 to 25 minutes. Cool completely on a wire rack.