Pour in whole milk and half and half. Whisk until well combined. Set heat on your stove to medium. As you heat the mixture continually whisk. Stir continuously until mixture is thickened and simmering.
Add about a cup of hot liquid to your slightly beaten eggs, adding 1 tablespoon at a time. Pour tempered eggs into your thickened custard while you whisk. Continue to stir and cook for an additional 2-3 minutes. Remove from heat and add in butter and vanilla. Stir until combined and smooth.