Preheat your oven to 350°F (175°C).
Line a 9x13 inch baking dish with parchment paper, leaving some overhang on the sides for easy removal later. Lightly grease the parchment paper.
In a large mixing bowl, combine the melted butter, light brown sugar, granulated sugar, and vanilla extract. Whisk until smooth and creamy.
Add in the large egg and whisk until fully incorporated.
In a separate bowl, mix together the all-purpose flour, baking soda, and a pinch of salt. Gradually add this dry mixture to the wet ingredients, stirring gently with a spatula until just combined.
Gently fold in the mini Reese's Peanut Butter Cups into the cookie dough.
Spread about two-thirds of the cookie dough mixture evenly into the prepared baking dish.
Drizzle the sweetened condensed milk over the cookie dough base, spreading it out evenly.
Crumble the remaining dough over the top of the sweetened condensed milk layer.
Bake for 25-30 minutes, or until the top is lightly golden and a toothpick inserted comes out clean.
Let the bars cool completely in the baking dish before lifting them out and slicing.
