1. Preheat oven to 350°. Generously butter 8 muffin cups.
2. Microwave 1/2 cup butter in a heatproof bowl to melt. Add chocolate, stirring until melted, then add sugar and cocoa and stir to blend. Whisk in eggs, vanilla, and salt. Add flour and stir until smooth. Spoon batter evenly into muffin cups.
3. Using a small spoon, make a depression in each portion of batter 1 in. wide and 1/2 in. deep. Spoon 1 tbsp. chocolate hazelnut spread into each.
4. Bake until a toothpick inserted into brownie part comes out with just a few moist crumbs, 20 to 24 minutes. Let cool on a rack about 10 minutes. Using a small metal spatula, loosen brownies from pans, move to rack, and cool completely.
Make ahead: Up to 1 day.
Note: Nutritional analysis is per brownie.