Preheat oven to 350F. Line an 8X8 pan with parchment paper.
In a large bowl combine melted butter and brown sugar. Mix with a nonstick spatula until combined. Add in eggs + vanilla and mix again. In a separate smaller bowl, combine flour, salt, and baking powder. Add dry ingredients to wet ingredients and mix until combined. Pour into the lined 8X8 pan. Place in oven. Bake for 20-25 minutes until the edges are browned and the center looks slightly underdone.
Make the Frosting
In a large bowl, pour the cereal and the heavy cream. Let cereal soak for at least 30 minutes longer the better!
Using a sieve, strain the heavy cream from the cereal into a large bowl.
Add confectioner's sugar to the strained heavy cream and beat using an electric mixer until stiff peaks form
Spread using an offset spatula onto cooled blondies
Scatter marshmallows on top and cut into squares!