Preheat the oven to 350°F. Lightly grease a 6'' cast iron skillet with butter and set aside.

In a large bowl, sift together the flour, cocoa powder, and salt. Set aside.

In a microwave-safe bowl, melt the butter. Add the granulated and light brown sugar to the butter, whisking until well incorporated. Crack the eggs in one at a time, then add the vanilla. Whisk the mixture until it is smooth and has lightened slightly in color.

In a small bowl, melt the chocolate in the microwave (or over a double boiler). Pour the melted chocolate into the egg and sugar mixture, then whisk well.

Pour the dry ingredients into the wet ingredients, add in the chocolate chips, and fold everything until just combined.

Pour the brownie batter into a 6'' cast iron skillet and bake in the oven for 35-40 minutes. If you insert a toothpick into the center of the brownie, it's okay if there's some brownie residue, as it will continue cooking in the pan.

Remove the skillet from the oven and let cool for 5-10 minutes. Top with flaky salt and ice cream. Serve immediately.