Preheat the oven to 375°F. Lightly butter a 10-inch skillet and set aside.
In a large bowl, add the melted butter, brown sugar, and granulated sugar and whisk until everything comes together.
Add in the egg and egg yolk and whisk vigorously until the eggs are well incorporated. Then whisk in the vanilla extract, maple syrup, baking soda, and salt.
Add in the flour and mix until it is just partially incorporated. Then add in your mix-ins and stir until the dough just comes together.
Spoon the dough into your prepared skillet and spread into a relatively even layer. Bake for 19 – 22 minutes or until the edges are set, but the middle still looks slightly underdone.
Place on a wire rack and let cool for a few minutes (or however long you can wait). Then slap on some ice cream, break out the spoons, and dig in!
This can also be made as individual or smaller skillet cookies. For a 6-inch skillet or smaller drop the time to about 15 minutes.