Beat all the cinnamon butter ingredients together. Store in a covered container in the refrigerator. Take out 1 hour before serving.
For the Popovers
Preheat oven to 450 F. Butter the cups of a 12 cup muffin tin. Butter it heavily, bottom, sides and around the rim. Set aside.
In a small bowl, whisk together the sugar, flour and salt. Set aside.
In a medium bowl, whisk the eggs and vanilla. Add in milk and melted butter and whisk gently until combined. Gently whisk in the dry ingredients until combined and lumps are gone. Should be the consistency of pancake batter.
Fill muffin cups about 2/3 full. If you have any empty cups fill them halfway with water to avoid scorching the pan.
Place in preheated oven for 15 minutes. Do NOT open the oven door. Reduce the temperature to 350 and bake another 15-20 minutes. They should be dark brown and crusty on the outside.
Place muffin pan on a cooling rack, you should see steam escaping from the wells of the popovers. If you don't, you may need to gently pierce the side of the popover with a sharp knife. Allow to cool for ten minutes then serve with cinnamon butter.
Notes
VARIATIONS: Traditional popovers: leave out the vanilla and the sugar. Herbed Popovers: Replace the melted butter with melted herb butter, and/or add 1 tablespoon of finely chopped fresh herbs to the batter. Garlic & Cheese: Add 2 tablespoons of finely shredded cheese and replace the melted butter with melted garlic butter. Fruit & Almond Popovers: Follow the sweet recipe adding in 1/2 teaspoon of almond extract with the vanilla. Serve with fruit jam or apple butter instead of cinnamon butter. Sugar Crusted: Another twist would be to brush baked popovers with melted butter and roll in sugar or cinnamon sugar (David Lebovitz http://www.davidlebovitz.com/2010/01/sugar-crusted-popover-recipe/). Chocolate: Replace 3 tablespoons of the flour with 3 tablespoons of unsweetened cocoa and follow sweet recipe above. Drizzle with melted chocolate and/or a powdered sugar glaze.