Line an 9-inch square baking pan with parchment paper or aluminum foil, leaving an overhang on the sides to lift the finished dessert out.
In a large mixing bowl, whisk to combine the dry ingredients: flour, baking powder, baking soda, and salt. Set aside.
In a separate bowl, blend the melted butter, brown sugar, and vanilla together until combined. Add egg and mix well.
Slowly add the dry ingredients into the bowl of wet ingredients and stir to combine. Fold in the walnuts. NOTE: The batter will be very thick.
Spread batter evenly into prepared baking pan using a rubber spatula. If mixture is too sticky to spread, dip spatula in water and try again.
Bake for 20 minutes or until inserted toothpick comes out clean. Remove from the oven and allow to cool completely before spreading the Rocky Road on top.
Rocky Road Topping
In large, uncovered, microwave-safe bowl melt chocolate and sweetened condensed milk on HIGH (100%) power for 1 minute. Stir until smooth.
If needed, microwave at additional 10- to 15-second intervals, stirring until completely melted. Stir in vanilla extract. Fold in marshmallows and walnuts.
Spread evenly over cooled, baked blonde brownies. Refrigerate until ready to eat then slice into squares.