Pre-heat oven to 350℉. Combine chocolate chips and butter in a medium sized glass bowl.
1 1/2 cups semi-sweet chocolate chips, 3/4 cup unsalted butter
Microwave on high until chocolate is melted. Stir until smooth. Allow it to cool for a few minutes while you make the batter.
Whisk together flour, cocoa, baking powder, and salt.
2 cups cake flour, 1/4 cup unsweetened cocoa, 1 1/2 tsp baking powder, 1/2 tsp kosher salt
Add eggs, vanilla, granulated sugar, and brown sugar to the bowl of a heavy-duty stand mixer. Add whisk attachment to mixer.
4 eggs, 1 tsp vanilla extract, 1 cup granulated sugar, 1 cup brown sugar
Set mixer to medium speed, and mix until batter is thick and creamy - about 5 minutes.
Reduce mixer speed to medium-low. Alternate adding flour mixture and melted chocolate a little at a time, and mix together until just incorporated.
Pour batter into a 12-inch well seasoned cast-iron skillet. Add skillet to pre-heated oven, and bake for 40 minutes.