Important: Let eggs and milk come to room temperature before you start cooking. The popovers will not rise properly without this step.
Preheat oven to 450°F / 232°C.
Place a 12-cup muffin pan in the oven as it preheats.
In a large mixing bowl, whisk eggs well. Whisk in milk, sugar, salt, and oil.
Add flour to the mixture in three additions, whisking gently after each addition until just combined. (Be careful not to overmix - it’s okay if some small lumps remain.
Remove hot muffin pan from the oven and carefully (the pan is very hot!) place it on a large rimmed sheet pan. (Note: This is optional, but useful to catch any drips that might occur during transferring / baking.)
Brush the insides of the muffin spaces with oil. (You can do this with a pastry brush or by dipping a clean paper towel in the oil and rubbing it in the muffin tin.)
Pour batter into muffin tin spaces right up to the top (you may have a small amount of batter left after the pan is full; discard any extra).
Return pan to oven. Reduce oven temperature to 400°F / 204°C. Bake until popovers aren golden brown and very puffy, about 33-35 minutes.
While the popovers bake, mash together ingredients for cinnamon butter - softened butter, sugar, and ground cinnamon.
Serve popovers warm, with cinnamon butter.