Brown the Butter: In a light-colored pan, melt the butter over medium-high heat, stirring frequently. Cook for about 5 minutes until brown bits form at the bottom. Remove from heat and let cool for 40-45 minutes.
Mix Wet Ingredients: In a mixing bowl, combine the cooled brown butter, granulated sugar, and brown sugar. Mix until light and fluffy. Add the egg, then the egg yolk, followed by the vanilla extract. Mix until fully incorporated.
Combine Dry Ingredients: In a separate bowl, whisk together the flour, salt, and baking soda. Gradually mix the dry ingredients into the wet ingredients until just combined. Do not overmix.
Assemble the Cookies: Scoop 80g of dough and flatten it in your palm. Place a Reese’s Thin in the center and wrap the dough around it, sealing the edges.
Add Toppings: Press the top of each dough ball into Reese’s Pieces.
Chill the Dough: Place the cookie dough balls on a baking sheet and refrigerate for at least 1 hour (up to 72 hours for enhanced flavor).
Bake: Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper. Place 4 cookie dough balls on the sheet and bake for 13-14 minutes, until the edges are golden but the center remains soft.
Cool & Enjoy: Let the cookies cool on the baking sheet for 3-4 minutes before transferring them to a wire rack. Serve warm for the ultimate gooey experience!