Melt butter over medium heat. Add the brown sugar and whisk until it melts and mixture begins to boil, about 5 minutes. Stir in milk, vanilla, and hazelnuts until smooth. Set aside.
Recipe by the modern barista at https://themodernbarista.com/index.php/2015/07/21/brazil-santa-ines-paired-with-plantain-stuffed-beignets/