Combine 2 tablespoons butter and peanut butter in a small saucepan. Heat contents over low heat and stir until well combined and butter is completely melted.
Place marshmallow fluff in the mixing bowl. Add peanut butter/butter mixture and mix until completely incorporated; set aside.
In a saucepan over low heat, melt the chocolate and remaining 4 tablespoons of butter. Mix well.
Remove from heat and stir in the sugar, egg, salt, flour, and vanilla; pour half of the brownie mixture into the baking dish. Spread so that the whole bottom of the dish is completely covered.
Place about 9 dollops of peanut butter/fluff mixture on top of brownie layer. Spread gently and cover entire chocolate layer.
Again, using dollops, place the brownie mixture on top of the pb/fluff. Spread gently so that the batter touches all the edges of the dish.
Run a knife through the brownies to somewhat combine the layers.
Place baking dish in oven and bake for 50 minutes.