*To make this skillet into snickerdoodle cookies, see the recipe notes below.
Preheat the oven to 350°F. Spread the softened butter all over the bottoms and sides of a 10-inch skillet. Set aside.
1-2 Tbsp. softened butter (to grease the skillet)
Brown the butter: For a detailed tutorial, see this post on how to brown butter. Allow the butter to cool slightly, ~10-15 minutes. 1 cup unsalted butter, cut into 1 tbsp. pieces
Add the sugars to the cooled browned butter and whisk until combined. Some of the mixture may get stuck in the whisk, just give it a tap to get it all out.
1 cup granulated sugar, 3/4 cup dark brown sugar, packed
Add the egg, egg yolk, milk, and vanilla extract. Whisk to combine until smooth. Be sure to break up any large clumps of brown sugar if you find any.
1 large egg + 1 egg yolk, room temperature, 2 Tbsp. whole milk, 1 Tbsp. vanilla extract
Add in the dry ingredients and fold just until combined. The cookie dough will be thick!
2 2/3 cups all-purpose flour, spooned and leveled, 2 tsp. cream of tartar, 1 tsp. kosher salt, 1 tsp. baking soda
Turn the cookie dough out onto prepared pan. Press cookie down into an even layer.
Combine the cinnamon and sugar topping together and sprinkle some over the tops. You don't have to use all of it, you can reserve some for serving.
1 Tbsp. sugar, 1/2 tsp. ground cinnamon
Bake for ~30-35 minutes or until mostly set and tops are golden brown. The middle may sink as it cools, that's just the nature of this recipe...you didn't mess up!
Allow to cool for ~30-45 minutes before cutting into wedges (errrr, if you can wait) and then serve with vanilla ice cream. If you slice into the cookie skillet too soon, it might look raw, so that cool time helps the crumb set up. I've found the cookie is chewy and delicious once it has cooled completely, then I'll pop a slice in the microwave and enjoy smothered with ice cream!
vanilla ice cream for serving (some would say optional, but I strongly encourage it 😉)