Preheat oven to 350℉. Lightly butter a 6-inch cast-iron or ovenproof skillet.
In a medium bowl, whisk together melted butter, brown sugar, and granulated sugar until smooth. Add in egg yolk and vanilla extract. Mix until combined, scraping the bowl as needed.
Add the flour, baking soda, and salt, mixing until just combined.
Gently fold in chocolate chips or chunks. The dough will be thick and soft.
Spread the batter onto the skillet, pressing it evenly to the edges.
Bake for 20-22 minutes, until edges are golden and the middle is set.
Remove from the oven and immediately sprinkle with flaky salt, if using.