Line a baking sheet with parchment paper or a silicone baking mat, then set aside. Heat-treat your brownie mix by pouring it into a microwave safe bowl and microwaving for 90 seconds, stirring every 30 seconds, or until it reaches 160 degrees F. Set aside.
In a microwave safe bowl, combine white chocolate chips with 2 tbsp vegetable oil. Microwave in 30 second intervals, stirring in-between, until melted and smooth.
Pour the melted white chocolate onto your prepared baking sheet and spread into a rectangle that’s roughly 9x13 inches in size. Transfer to the fridge or freezer to harden for about 15 minutes.
While the chocolate sets, combine your heat-treated brownie mix with softened butter and milk. Mix to form a batter consistency, then evenly spread the brownie batter on top of the chilled chocolate layer.
In another microwave safe bowl, combine semi-sweet chocolate chips with 1 tbsp vegetable oil. Microwave in 30 second intervals, stirring in-between, until melted and smooth, then spread the melted chocolate on top of the brownie batter.
Immediately sprinkle with mini chocolate chips, then return to the fridge or freezer to chill for another 15-30 minutes. Break or cut into pieces before enjoying!