Preheat the oven to 350 degrees. Line an 8×8 baking pan with parchment paper and lightly spray with oil.
In a medium saucepan, melt butter over medium heat.
Once butter has melted, remove from heat. Add chocolate chips and cocoa powder and whisk to combine until fully melted and shiny. Set aside to cool.
In a stand mixer fitted with a whisk attachment, beat the eggs and sugars on medium high speed until light and creamy. The color should be a light white/yellow. About 3-4 minutes.
Once the butter and chocolate mixture has cooled slightly, slowly stream into the egg and sugar mixture while on medium speed. Careful to slowly add the chocolate during this step. If the chocolate is still too warm, it can scramble the eggs. Beat for about 30 seconds or until fully incorporated.
Remove the bowl and fold in the flour using a wooden spoon or spatula. Careful to not over mix during this step.
Pour the batter into the prepared pan. Bake on the center rack of the oven for 35-45 minutes or until a toothpick inserted in the center comes out with just a few crumbs.
Remove the pan from the oven and let it cool completely on a wire cooling rack before slicing.