In a small saucepan combine milk, 1/4 cup sugar, cinnamon, lemon zest, and salt. Bring to a simmer. Meanwhile, in a medium bowl whisk together yolks, cornstarch, and remaining 1/4 cup sugar.
When milk gets to a simmer, slowly ladle about 1/2 cup of milk into yolk mixture, whisking constantly. In a slow stream, continue to pour in milk, whisking constantly. Return mixture to saucepan over medium heat and cook until thickened, about 3 minutes, stirring constantly.
Remove from heat and whisk in butter and vanilla. Cover with cellophane directly on top of the creme and refrigerate 2 hours or up to 2 days.