Start by lining a 9x9-inch baking pan with parchment paper for easy removal of the bars once set.
In a large mixing bowl, combine the softened unsalted butter and packed light brown sugar. Cream together with a spatula until light and fluffy, about 2-3 minutes.
Stir in the vanilla extract until fully incorporated to enhance the flavor.
Gradually add the all-purpose flour to the butter and sugar mixture. Mix until combined, being careful not to overwork the dough.
Pour in the sweetened condensed milk and stir until the mixture is thick and creamy.
Gently fold in the mini chocolate chips, ensuring even distribution throughout the dough.
In a separate bowl, mix the creamy peanut butter until smooth. Warm slightly if needed for easier mixing. Fold it into the cookie dough mixture until just combined.
Spoon the cookie dough mixture into the prepared baking pan. Press down firmly and evenly to create a uniform layer.
Sprinkle the milk chocolate chips over the top of the bars, pressing them gently into the dough for added texture and sweetness.
Cover the pan with plastic wrap and refrigerate for at least 2 hours or until the bars are firm.