In a medium bowl, whisk together the flour, baking soda, and kosher salt and set aside. In a small bowl, whisk together the egg and vanilla extract and set aside.
In a stand mixer fitted with the paddle attachment, cream the butter and sugar until fluffy, approximately 2 minutes. Add the egg mixture and mix on low speed until just combined.
Add the flour mixture and mix on low speed until just combined. Be careful not to overbeat. Remove the bowl from the mixer and fold in the chocolate and Marcona almonds with a spatula. Divide the dough into 2 portions.
Lightly butter an 8-inch cast-iron skillet and press down half of the dough so that it is evenly flat in the skillet. Chill in the refrigerator for at least 20 minutes and up to 2 days (covered).
When ready to bake, preheat the oven to 350°F.
Bake for 20 to 25 minutes. Around the 7-minute mark, pull the pan out of the oven and bang it on the countertop to flatten the cookie, then sprinkle the finishing salt on top and place the pan back into the oven. Remove when the cookie looks set but not overbaked for a chewy center. Let cool for 5 minutes, then transfer to a wire rack.