Preheat oven to 350°F. For brown butter, in a medium skillet heat butter over medium-low until melted. Simmer, stirring or swirling the skillet occasionally, until butter turns golden brown and smells nutty, about 5 minutes. Transfer butter to a large bowl; cool slightly.
In a medium bowl whisk together flour, baking soda, cinnamon, and ¾ tsp. salt.
Whisk brown sugar, granulated sugar, eggs, and vanilla into brown butter. Stir in flour mixture. Stir in chocolate chips, oats, coconut, and pecans until combined. Press dough evenly into a 12-inch cast-iron skillet. Sprinkle top with sea salt flakes.
Bake until cookie is firm to the touch and edges begin to pull away from sides of skillet, 35 to 40 minutes. Run a thin metal spatula around edge of cookie to loosen it. Cut into wedges and serve slightly warm or at room temperature with ice cream and additional coconut flakes, if you like. (Cover cookie and store at room temperature up to 3 days.)
