Preheat the oven to 350 degrees F (177 degrees C).
Prepare Cinnamon Sugar: In a small bowl, mix sugar and ground cinnamon; set aside.
Combine Dry Ingredients: In a medium-sized bowl, stir together flour, sugar, baking powder, and salt.
Mix Wet Ingredients: In another bowl, combine the egg yolk, milk, melted butter, and vanilla (if using).
Combine Wet and Dry: Pour the wet ingredients into the flour mixture and stir until just moistened.
Prepare Ramekinand Bake: Lightly grease a 10-ounce ramekin and pour in the batter. Bake for 23 to 28 minutes. It typically takes me 25 minutes.
Add Topping: Once baked, you have two options:Drizzle ½ tablespoon of melted butter over the top of the muffin and sprinkle with the cinnamon-sugar mixture while it’s still in the ramekin - ORUse a knife to loosen the sides of the muffin from the ramekin, transfer it to a plate, and then drizzle with melted butter and sprinkle with the cinnamon-sugar mixture.
Enjoy while still warm.
Notes
Preheat your oven: A preheated oven ensures your muffin will cook evenly and rise beautifully.
Don’t overmix the batter: A few lumps are perfectly fine. Overmixing can lead to a tough muffin.
Use a 10-ounce ramekin: The size of the ramekin you use is incredibly important. This size ensures the batter bakes evenly and rises beautifully.
Brush with melted butter before adding the cinnamon sugar: This helps the topping stick to the muffin.