Preheat oven to 350 degrees. Generously butter eight muffin cups in a tin.
Microwave 1/2 cup butter in a heatproof medium 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.
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.
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.
Serve with or without a scoop of vanilla bean ice cream—and don’t forget the Port!