Gradually stir half of the mixture into egg yolks to temper. Gradually add egg yolk mixture back to milk mixture in saucepan. Bring to bubbly simmer and cook, stirring constantly, for 1 minute.
Remove from heat and stir in butter and vanilla. Pour into bowl, cover with plastic wrap to prevent a skin from forming, and refrigerate at least 1 hour until cool.
This pastry cream recipe is adapted from the Betty Crocker Cookbook.