Ingredients

• 1 stick (1/2 cup) unsalted butter, cut into pieces
• 2 ounce unsweetened chocolate, chopped
• 1 cup sugar
• 2 large eggs
• 1/2 teaspoon pure vanilla extract
• 2/3 cup all-purpose flour
• 8 ounces cream cheese, well softened
• 1/3 cup sugar
• 1 large egg yolk
• 1/4 teaspoon pure vanilla extract

Instructions

1. Put oven rack in middle position and preheat oven to 350°F. Butter an 8-inch square baking pan.
2. Heat butter and chocolate in a 3-quart heavy saucepan 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.
3. Whisk together cheesecake batter ingredients in a small bowl until smooth. Dollop over brownie batter, then swirl in with a knife or spatula.
4. Bake until edges are slightly puffed and center is just set, about 35 minutes.
5. Serve warm or at room temperature.