Melt unsalted butter completely, then set aside to cool for at least 10 minutes.
While the butter is cooling, add flour, cocoa powder, cornstarch, baking soda, and salt to a medium bowl. Whisk to combine, then set aside.
Add light brown sugar and granulated sugar to a large bowl, then add melted butter and stir to combine.
Add egg, egg yolk, and vanilla extract and beat until well-combined.
Add dry ingredients to wet ingredients a little at a time, stirring until just combined.*