In a small bowl, combine the flour, baking powder, cinnamon, salt, and brown sugar.
In a separate bowl, whisk together the butter, egg yolk, milk, and vanilla.
Stir the wet ingredients into the dry ingredients.
Fold in the diced apples and chopped pecans.
Transfer the batter to a lightly buttered 10-ounce ramekin.
Make the topping
Combine the flour, brown sugar, cinnamon, chopped pecans, and softened butter in a small bowl until crumbly.
Pour the streusel topping over the muffin batter and spread evenly over the top.
Bake for 25-30 minutes or until a toothpick inserted into the center of the muffin comes out clean.
Cool for 5 minutes on a wire rack and enjoy warm.
Notes
Before you start cooking, make sure to read the entire recipe, including the Ingredient Notes section, and examine the process photos. This detailed guide will help you understand how the recipe unfolds, ensuring your apple muffin turns out perfect every time. All the information you need is found within the body of this recipe post, so don’t miss a thing!
Choose the Right Baking Dish: The recipe's success hinges on the correct baking dish. Opt for a 10-ounce ramekin that has a 4-inch diameter and a height of around 1 ¾ inches. Using a larger dish will thin out the batter, reducing cooking time, while a smaller dish will result in a thicker muffin that may need longer in the oven.
Use a Rimmed Baking Sheet: To prevent any messy spills, place your ramekin on a rimmed baking sheet before popping it into the oven.
Avoid Overmixing: Stir your batter just enough to combine the ingredients. Overmixing can lead to a tougher muffin texture.
Accurate Flour Measurement is Key: Spoon the flour into your measuring cup until it overflows, then level it off using the flat edge of a kitchen knife. This ensures you're using the exact amount needed for a perfect muffin.