Whisk sugar, cornstarch, and salt together in medium saucepan.
Add milk and egg yolks, whisk to combine.
Bring to boil over medium heat just until boiling, whisking or stirring constantly.
Continue to cook until thick (like pudding) remove from heat and stir in butter and white chocolate chips until melted - this may take a few minutes.
Pour into bowl or other container and lay plastic wrap across the surface, being sure it is touching the surface (to prevent a skin from forming).
Place in refrigerator for several hours, or until chilled.
Once chilled you can remove the plastic wrap and just use the container's lid to cover.