Taking brownies to a whole new level. These are a bit more labor intensive than your every day brownies, but I love to make them for special occasions. This recipe can be doubled (but don't double the egg in the filling) and baked in a 9x13 pan, adding about 5 minutes to the baking time.
Preheat oven to 350 deg F. Spray an 8x8 baking pan with cooking spray.
In a small bowl, combine all filling ingredients and beat until smooth; set aside.
In a microwave dish, melt butter and chocolate. Cool slightly. In a separate bowl, lightly beat eggs and vanilla. Very slowly drizzle the chocolate mixture into the eggs while whisking quickly to prevent eggs from cooking. Stir in remaining batter ingredients until blended.
Spread batter in an 8x8 pan. Then spread filling over the batter. Sprinkle the topping nuts over the filling; then sprinkle the chocolate chips on top. Bake for 23 minutes or until almost set (middle will jiggle a bit like a cheesecake). Sprinkle with marshmallows and bake for 2 more minutes.
Heat all frosting ingredients in a large pan, whisking until everything is melted. Remove from heat and whisk in vanilla and powdered sugar. Drizzle over marshmallows, covering as much area as possible. Use a small spatula to spread the frosting to cover the entire top. Cool completely and then refrigerate overnight. Cut into bars.