Preheat your oven to 350°F (175°C) and line your 9x13 inch baking pan with parchment paper, allowing some overhang for easy removal.
In a medium saucepan, melt the butter over low heat. Remove from heat and let cool slightly.
In your mixing bowl, combine the melted butter with granulated sugar and light brown sugar. Whisk until smooth.
Add the eggs one at a time, whisking thoroughly after each addition. Stir in the vanilla extract.
In a separate bowl, whisk together the all-purpose flour, baking powder, and kosher salt. Gradually add to the wet mixture, stirring gently.
Fold in the chopped chocolate until evenly distributed.
Pour the batter into the prepared baking pan and spread evenly. Bake for 25-30 minutes until edges are set.
Remove from the oven and let cool in the pan for about 10 minutes. Lift out using parchment and cool completely before slicing.
