Preheat oven to 350°F. Line an 8″x8″ dish with parchment paper and set aside.
In a microwave-safe bowl, combine butter and white chocolate chips and microwave for 30 seconds. Stir the mixture until completely melted (if not melted, give it another 10-15 seconds). It may be al little lumpy, but don't worry - the mixture will combine well with the next ingredients to become smooth.
Add in egg, sweetened condensed milk, vanilla extract, and salt, stirring to combine. Fold in flour, stirring until just combined.
Pour mixture into prepared pan and bake for 22-25 minutes. It should be puffed up in the middle. This will sink back down when you remove it from the heat. Remove from oven, allow to cool in pan for 10 minutes, then transfer the brownies out of the pan and onto a cooling rack. Allow to cool completely before serving.
Notes
It's important to slightly underbake these as they will continue to cook in the pan after they come out of the oven. This recipe is from Broma Bakery, and it's absolutely delicious, in addition to being very simple to make! Thanks Sarah! https://bromabakery.com/white-chocolate-brownies-recipe/