Place the milk in a saucepan and bring to a scald. While milk is heating, whisk the yolks and sugar together by hand until well combined but not frothy. Gently stir in the cornstarch and vanilla extract. Temper the hot milk into the yolk mixture with the whisk. Add the mixture back into the saucepan and bring to a boil again, whisking constantly. Continue to whisk vigorously and boil for one minute or more until well thickened and no taste of starch remains. Remove from heat. Strain mixture into a clean container and stir in the butter. Cover the cream with plastic wrap directly on its surface, poking a few small holes in the plastic so steam can escape. Cool completely.
Fold in 10oz of butterscotch chips for butterscotch custard.