16 ounces cream cheese , at room temperature
1 3/4 cups sugar
3/4 teaspoon salt
2 large egg whites , at room temperature
2 tablespoons plus 3/4 cup sour cream , at room temperature
1/3 cup miniature semisweet chocolate chips
1 1/2 cups all-purpose flour
1/2 cup Dutch-processed cocoa powder
1 1/4 teaspoons baking soda
1 1/3 cups water
8 tablespoons (1 stick) unsalted butter , melted and slightly cooled
1 teaspoon vanilla extract
1. Adjust oven rack to lower-middle position and heat oven to 400 degrees. Line 2 standard muffin tins with cupcake liners.
2. With electric mixer on medium speed, beat cream cheese, 1/2 cup sugar, and 1/4 teaspoon salt in medium bowl until smooth, about 30 seconds. Beat in egg whites and 2 tablespoons sour cream until combined, about 1 minute. Stir in chocolate chips and set aside.
3. Whisk remaining sugar, remaining salt, flour, cocoa, and baking soda in large bowl. Make well in center, add remaining sour cream, water, butter, and vanilla and whisk until just combined. Divide batter evenly among 24 cupcake liners and top each batter with 1 rounded tablespoon cream cheese mixture. Bake until tops of cupcakes just begin to crack, 23 to 25 minutes. Cool cupcakes in tins for 10 minutes before transferring to wire rack to cool completely. (Cupcakes can be refrigerated in airtight container for up to 2 days.)