Line an 8x8 inch pan with parchment paper and set aside.
In a large microwave-safe bowl, add the white chocolate chips, sweetened condensed milk, vanilla extract, and butter and melt in the microwave in 30-second increments. Make sure to stir in between every time.
Add in the heat-treated sugar cookie mix and stir until combined. Mix in about 3 tablespoons of your sprinkles of choice then spread into the baking pan. Top with extra sprinkles then chill in the fridge for 2-3 hours before slicing and enjoying.
Notes
The quickest and easiest way to heat treat your cookie mix is to microwave it in 30-second increments until it reaches 160F. You can alternatively place it in a sheet pan lined with parchment paper and bake in a 350F oven for 5-7 minutes or until it reaches 160F.