Prepare and bake the brownie mix according to the package instructions.

While the brownies are baking, add the butter and peanut butter chips to a saucepan over medium low heat. Whisk to combine until the mixture is completely melted and smooth.

Remove the mixture from the heat and stir in the vanilla and sweetened condensed milk.

Pour the mixture over the brownies and let cool until set completely.

Slice and serve.