Line an 8x8-inch pan with parchment paper. In separate containers, measure out Dutch process cocoa powder, marshmallows, chocolate chips, vanilla, and salt (have them ready to stir in while still hot!). Set aside.
Mix C&H® Light Brown Sugar, evaporated milk, and butter in a large heavy bottom pan (such as a Dutch oven) over medium heat, stirring constantly. Bring to a roiling boil. Stir constantly while boiling for 5 mins.
Turn heat off, stirring Dutch cocoa powder until dissolved. Immediately add in marshmallows, chocolate chips, vanilla, and salt. Fold/stir until combined and all the Marshmallows are melted.
While still hot, pour ½ of the fudge mixture into the 8x8-inch pan and spread to edges. Spread the mix-in marshmallows throughout and cover with remaining fudge. Let cool for a few hours, if not overnight. Cut into pieces and enjoy!