1 stick unsalted butter
2 ounces unsweetened baking chocolate
1 cup sugar
2 eggs, at room temperature
1 teaspoon vanilla extract
1 teaspoon salt
1/2 cup all-purpose flour
6 ounces dark chocolate chunks

Heat oven to 350 F. In a small saucepan over medium heat, melt butter and baking chocolate. Remove from heat and stir in sugar, eggs, vanilla and salt. Whisk vigorously with wooden spoon until mixture is smooth and well combined. Stir in flour, then gently fold in dark chocolate chunks. Pour batter into an 8-inch-by-8-inch baking dish or a 10-inch cast-iron skillet. Bake 20 to 25 minutes, or until the top and edges are set but the center is still slightly gooey. Serve warm with whipped cream or ice cream.