Begin by heating your oven to 350 degrees Fahrenheit. Prepare a 9x13-inch baking pan by either greasing it or using parchment paper with a light grease.
In a large bowl, mix together the melted butter, granulated and brown sugars, egg, egg yolk, vanilla, baking soda, and salt. Stir until all ingredients are well incorporated.
Carefully fold in the all-purpose flour, oats, and chocolate chips until everything is combined evenly.
Pour the batter into the prepared baking pan, smoothing it out, and bake for 18 to 20 minutes, ensuring not to overcook; a slightly underbaked texture enhances the flavor. Serve warm with milk!