In a stand mixer fitted with the paddle attachment, cream the butter and both sugars together until light and fluffy, about 3 minutes. Add the vanilla, flour, salt, and milk, beating until combined. Stir in the chocolate chips; set aside.

Line a baking sheet with wax paper. In a microwave-safe bowl, combine the blue candy melts and the white chocolate. Heat in the microwave until smooth and glossy*, then spread half of the melted chocolate onto the lined baking sheet into a rectangle that’s roughly 8″ by 11″. Transfer to the refrigerator until set, about 2 minutes.

Once set, scoop cookie dough evenly over the chocolate and use a rubber spatula or your fingers to gently press the filling into an even layer.

If needed, reheat the remaining chocolate for another 15 seconds, or until smooth and pourable again. Drizzle on top of the cookie dough, using an offset spatula or knife to spread evenly and ensuring that all cookie dough is covered. Immediately add the toppings. Chill until set, about 20 minutes. Break/Cut into pieces and serve.