Cheesecake Marbled Brownies

Adapted from Smitten Kitchen

Brownie batter
1 stick (1/2 cup or 4 ounces) unsalted butter, cut into pieces
3 ounces unsweetened chocolate, chopped
1 cup sugar
2 large eggs
1/2 teaspoon pure vanilla extract
2/3 cup all-purpose flour

Cheesecake batter
8 ounces cream cheese, well softened
1/3 cup sugar
1 large egg yolk
1/4 teaspoon pure vanilla extract

Plus
1/2 cup semisweet chocolate chips

Make brownie batter: Put oven rack in middle position and preheat oven to 350°F. Butter an 8-inch square baking pan. Heat butter and chocolate in a 3-quart heavy saucepan — though I did mine double-boiler style, placing the mixing bowl I was using over simmering water, thereby creating one less dirty dish and melting the chocolate more gently — over moderately low heat, whisking occasionally, just until melted. Remove from heat and whisk in sugar, eggs, vanilla, and a pinch of salt until well combined. Whisk in flour until just combined and spread in baking pan.

Make cheesecake batter: Whisk together cheesecake batter ingredients in a small bowl until smooth. Dollop over brownie batter, then swirl in with a knife or spatula.

Bake brownies: Bake until edges are slightly puffed and center is just set, about 35 minutes. Serve warm or at room temperature.