Instructions

• Preheat oven to 350°F (175°C). Spray an oven-safe skillet with cooking spray; if using well‑seasoned cast iron, butter or grease it lightly instead. Set the skillet aside.
• Place ½ cup unsalted butter in a microwave-safe bowl and melt on high for about 1 minute. (Optional: to make browned butter, microwave 3–4 minutes, watching closely so it doesn’t burn.)
• To the melted butter, add ¾ cup packed light brown sugar, 1 large egg, and 2 teaspoons vanilla extract. Whisk vigorously for about 1 minute until combined and slightly fluffy.
• Add ¾ cup all-purpose flour, ½ teaspoon baking soda, and ½ teaspoon salt (optional, to taste). Stir until just incorporated.
• Add 1½ cups old‑fashioned whole rolled oats (not quick cook or instant) and stir until evenly combined.
• Transfer slightly less than three-quarters of the dough into the prepared skillet, pressing it into an even layer. Reserve about 1¼ cups of dough in the bowl to use as the crumble topping (eyeball this amount).
• In a small bowl, combine one 14-ounce can sweetened condensed milk and ½ cup creamy peanut butter. Stir until smooth and well combined.
• Pour the sweetened condensed milk–peanut butter mixture evenly over the dough in the skillet.
• Sprinkle 1 cup semi-sweet chocolate chips evenly over the peanut butter layer.
• Break or drop the reserved dough into tablespoon-sized chunks and distribute them evenly over the top to form a crumble topping.
• Bake for about 25 minutes, or until the edges begin to barely crisp and most of the top surface has set. The center may still be gooey—this is expected because the skillet will continue to cook from residual heat. Do not overbake if you want a very gooey center.
• Remove the skillet from the oven and let it rest briefly (the cookie will finish setting). Serve warm, optionally topped with ice cream.