Dark Chocolate Sauce

1 cup packed light brown sugar
2/3 cup unsweetened cocoa powder
1 1/2 cups 1% milk
2 ounces chopped bittersweet or semisweet chocolate
1 teaspoon vanilla extract

Whisk brown sugar and cocoa powder in a large heavy saucepan.

Gradually whisk in milk until a smooth paste forms. Bring to a simmer, whisking constantly. Reduce heat to low and simmer, whisking constantly, for 4 minutes.

Remove from heat and add chopped chocolate; stir until melted. Stir in vanilla.
Let cool slightly; serve warm.