Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the melted light butter, natural peanut butter, brown sugar, sugar (or sweetener), and vanilla extract. Whisk until smooth. Add the egg and mix until well incorporated.
In a separate bowl, whisk together the flour and salt. Gradually add this dry mixture to the wet ingredients and stir gently until just combined. Do not overmix.
Gently fold in the milk or dark chocolate chips until evenly distributed throughout the dough.
Lightly grease your oven-safe skillet to prevent sticking. Spread half of the cookie dough evenly in the bottom of the skillet to create a base layer.
Drizzle the melted Nutella over the base layer of cookie dough. Sprinkle the quartered mini Reese's cups evenly over the Nutella.
Carefully spread the remaining cookie dough over the Nutella and Reese's layer, covering it completely. Use a spatula to smooth it out.
Bake the skillet cookie in the preheated oven for 25-30 minutes, or until the edges are golden brown and the center is set but still soft.
Remove the skillet from the oven and let it cool for a few minutes before serving. Enjoy warm, optionally with a scoop of ice cream.