Preheat an oven to 350°F and line a metal 9x9 pan with parchment paper.
Brownie Batter: In a medium-sized bowl, combine the warm melted butter and caster sugar. Add the whole eggs, egg yolk, and vanilla extract. Whisk vigorously by hand for a full 1 to 2 minutes until the mixture becomes smooth, and glossy.
Add the cocoa powder to the butter mixture. Whisk thoroughly until the cocoa is completely incorporated.
Add the flour, cornstarch, and kosher salt to the bowl. Switch to a rubber spatula and gently fold the mixture just until combined. Do not overmix.
Transfer to the prepared pan and gently spread out into an even layer. Bake for 15 minutes. Remove from the oven and allow to cool while making the pecan pie filling. Aim for 5-10 minutes of cooling before adding the filling.
Pecan Pie Filling: Combine the melted butter, brown sugar, corn syrup, eggs, vanilla extract, salt, pecans and chocolate chips in a medium bowl. Stir to combine.
Once the brownies have cooled for 5-10 minutes, pour the filling over the brownies. Return to the oven and bake for another 25 to 30 minutes. Or until the top is set.
Cool completely before slicing. If some of the liquid runs out when you slice it, it's okay. Enjoy!