Heat the oven to 350 degrees. Spray a 13x9-inch pan with cooking spray.
In a large bowl, mix the brownie mix, butter, sour cream, 2 eggs and 1 egg yolk until just mixed (about 50 strokes). Bake for 30 to 35 minutes or until set.
Immediately upon removal from the oven, top with the chocolate chips, mini marshmallows, and chopped pecans.