Preheat the oven to 350° F. Line a 9x9 pan with parchment paper and set aside.
In the bowl of a stand mixer or in a large bowl with an electric hand mixer cream together the melted butter, brown sugar, and granulated sugar for 3 minutes.
Add the eggs and vanilla extract, mixing until well combined.
Add in the flour, cocoa powder, cornstarch and salt. Mix until the batter comes together.
Pour into the prepared pan and spread smooth.
Bake at 350° F for 22-27 minutes or until a toothpick comes out mostly clean from the center.
Allow the brownies to cool completely and then place in the freezer. Once the brownies are cool and are in the freezer, make the peanut butter buckeye layer.