Arrange a rack in the middle of the oven and heat to 375°F.
Place the flour, baking soda, and salt in a medium bowl and whisk to combine; set aside.
Place the butter, brown sugar, and granulated sugar in the bowl of a stand mixer fitted with the paddle attachment. (Alternatively, use an electric hand mixer and large bowl.) Beat at medium speed until lightened in color and creamy, about 2 minutes. With the mixer on low speed, beat in the eggs and vanilla. Scrape down the side of the bowl. Beat in the flour mixture on low speed until just combined.
Set aside 1/3 cup of the chopped chocolate. Add the remaining chocolate and nuts if using, to the dough and fold to combine. Transfer the dough to a 10-inch cast iron or oven-safe skillet. With damp hands, press the dough in an even layer to the edges of the skillet. Sprinkle the reserved 1/3 cup chocolate over top and gently press into the dough.
Bake until the center is set and the edges are golden-brown and slightly pull back from the skillet, about 25 minutes. Set aside to cool for at least 30 minutes before cutting into wedges and serving.