Preheat the oven to 350 degrees Fahrenheit. Line a 9"x13" baking pan with parchment paper so it hangs over the sides.
In the bowl of an electric mixer, cream together melted butter, brown sugar, granulated sugar, vanilla and eggs. Cream it until the batter lightens in color and gets fluffy.
In a separate bowl, whisk together flour, baking cocoa, baking powder, salt and corn starch.
Add flour mixture to wet mixture and mix just until incorporated, scraping down the sides with a spatula in between.
Spread batter evenly into prepared pan. Bake for about 15-16 minutes, or just until set. Don't over bake or they'll be dry!
Let the brownies cool completely on a cooling rack. Use the parchment paper to carefully lift them from the pan.
After the brownies are cool, make the frosting by creaming together softened butter, powdered sugar, vanilla, orange food coloring (4-5 drops or enough to reach desired color), and just enough milk for thinning (1 Tbsp. at a time) in the bowl of an electric mixer. Cream it for at least 4-5 minutes until it's nice and fluffy, creamy and spreadable. Frost the cooled brownies and add Halloween sprinkles.
Slice brownies and enjoy!