For brownie batter

* 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

For cheesecake batter

* 8 ounces cream cheese, well softened

* 1/3 cup sugar * 1 large egg yolk

*1/4 teaspoon pure vanilla extract

Preparation: 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.

Whisk together cheesecake batter ingredients in a small bowl until smooth.

Dollop over brownie batter, then swirl in with a knife or spatula.

Bake until edges are slightly puffed and center is just set, about 35 minutes.