In a small bowl, combine flour, both sugars, butter, salt, and cinnamon. Mix using your fingers or a fork until it forms pea-sized clumps. Stir in the reserved diced apples, then refrigerate the streusel.

Preheat your oven to 425°F and position the rack in the middle. Line a muffin tray with 4 paper liners.

In a medium bowl, whisk together the sour cream, pumpkin purée, juice, both sugars, egg yolk, and vanilla until smooth. Gently fold in the apple cubes.

In another bowl, combine the flour, baking soda, salt, and pumpkin pie spice. Gradually mix the dry ingredients into the wet mixture until just combined and no dry spots remain. Avoid overmixing to keep the muffins tender.

Distribute the batter evenly among the muffin liners, then sprinkle the streusel topping over each one.

Bake the muffins at 425°F for 5 minutes to help the tops rise, then lower the temperature to 350°F. Continue baking for another 11–13 minutes, or until a toothpick comes out mostly clean.

Allow the muffins to cool in the tray for about 10 minutes, then transfer them to a wire rack to cool completely.

Store your muffins at room temperature in a covered container for 3 to 5 days.