01 -
In a small bowl, combine all-purpose flour, unsalted butter, light brown sugar, granulated sugar, salt, and ground cinnamon or pumpkin pie spice. Cut the butter into the flour using fingertips or a fork until the mixture holds small, pea-sized crumbles.
02 -
Gently fold in the reserved 1 1/2 tablespoons diced apple. Refrigerate streusel until needed.
03 -
Preheat oven to 425°F (218°C) and adjust rack to the center. Line a standard muffin pan with 4 paper liners.
04 -
In a medium bowl, combine sour cream, canned pumpkin, juice or water, granulated sugar, light brown sugar, egg yolk, and vanilla extract. Mix with a fork until smooth. Stir in apple cubes, reserving 1 1/2 tablespoons for the streusel.
05 -
In a separate bowl, whisk together all-purpose flour, baking soda, salt, and pumpkin pie spice.
06 -
Sprinkle dry ingredient mixture over the wet ingredients. Fold with a spatula just until no dry streaks remain, avoiding overmixing.
07 -
Evenly divide batter among prepared muffin liners.
08 -
Add equal portions of streusel topping to each muffin.
09 -
Bake at 425°F (218°C) for 5 minutes. Reduce temperature to 350°F (177°C) and bake an additional 11–13 minutes, or until a toothpick inserted in the center emerges nearly clean.
10 -
Cool muffins in the pan on a wire rack for 10 minutes, then transfer to a rack to cool completely before serving.