Preheat the oven to 300 degrees F. Butter and flour an 8-inch square pan.
In a mixer fitted with a whisk attachment or a hand mixer, beat the eggs at medium speed until fluffy and light yellow. Add both sugars. Add in chocolates, butter, vanilla, flour and salt. Mix well.
Pour the batter into a greased and floured pan 9x9 or larger. Microwave marshmallow cream to soften up the pour in lines or drop into brownie batter and swirl into brownie batter. Bake for 30-45 minutes depending on the size of your pan. Check for doneness with a toothpick inserted into the center and should come out clean. When it's done, remove to a rack to cool. Top with mini marshmallows and chocolate chips.