Line a 9x9 baking pan with parchment paper and preheat oven to 350F (177C).
Combine melted, cooled butter and brown sugar, stir well.
Stir in Nutella until well-combined.
Add eggs & egg yolk, one at a time, stirring very well after each addition, then stir in vanilla extract.
In a separate, medium-sized bowl, whisk together flour, corn starch, baking powder, and salt.
Gradually stir dry mixture into Nutella mixture until completely combined.
Spread into prepared baking pan.
With your 1/4 cup Nutella, drop dollops of Nutella over the batter. Use a butterknife to swirl over the surface.
Bake on 350F (177C) for 40-45 minutes (a toothpick inserted in the center of the pan should come out mostly clean with few dry crumbs, don't over-bake or these will be dry!). Allow to cool before cutting and enjoying.