For the Filling:
8 ounces cream cheese, regular or reduced fat, at room temperature
1/3 cup granulated sugar
1 large egg, at room temperature
2 ounces bittersweet or semisweet chocolate, coarsely chopped

For the cupcakes:
1 1/2 cups all-purpose flour
1 cup firmly packed light brown sugar
5 tablespoons natural unsweetened cocoa powder (not Dutch-process)
1 teaspoon baking soda
1/4 teaspoon salt
1 cup water
1/3 cup unflavored vegetable oil
1 tablespoon white or cider vinegar
1 teaspoon vanilla extract