Preheat oven to 350°. Grease an 8x8 or 9x9 baking pan. Removes the easiest by lining with parchment paper and sprayed with non-stick spray. Set aside.
In a large pot, melt the butter and chocolate over medium heat, stirring often.
Once melted, remove from heat and slowly add the sugar, stirring constantly.
Add eggs, one at a time, mixing well after each.
Add vanilla extract and stir.
Add in flour, cocoa powder and salt, stirring until combined. Add the chocolate chips, ½ C of marshmallows and the pecans. and stir to combine.
Top with the remaining ¼ C of marshmallows (or more, if you'd like) and bake in the oven for 35-40 minutes, or until the center springs back when touched lightly.
Cool in pan on cooling rack until cooled. Slice and serve!