Step 1
Position an oven rack on the lowest level and preheat the oven to 375°F. Roughly chop 1 cup of semisweet chocolate wafers, leaving a few whole pieces, and set aside.

Step 2
In a large bowl, combine the dark brown sugar, granulated sugar, butter, and salt. Mix until mostly smooth with no large lumps, about 1 minute. Add the egg and vanilla, stirring until fully blended and the mixture looks loose and batter-like. Mix in the flour and baking soda just until a soft dough forms with no dry spots. Fold in ¾ cup of the chopped chocolate, saving the rest for the top.

Step 3
Transfer the dough to a 10-inch cast-iron skillet (or a greased oven-safe stainless-steel skillet). Using damp hands, press the dough evenly across the pan, making sure it reaches the edges. Sprinkle the remaining chocolate over the surface and finish with flaky sea salt.

Step 4
Bake until the edges are golden brown and slightly puffed, about 20–25 minutes (or 15–18 minutes if using a stainless-steel skillet). Let the cookie cool in the skillet for about 1 hour before serving. Slice into wedges and enjoy straight from the pan or turned out onto a plate.