Add the chopped up white chocolate to a microwave safe bowl, top with the heavy cream, sweetened condensed milk, and evaporated milk, and microwave in 30 second intervals until fully melted, stirring in between every time.
Whisk vigorously to get everything emulsified properly and use in your white chocolate mocha drinks.
Notes
If you do not have a microwave you can make this white chocolate mocha sauce over a double boiler.