In a small bowl, combine flour, cold cubed butter, light brown sugar, granulated sugar, a pinch of salt, and ground cinnamon. Mix until resembling pea-sized crumbs. Fold in reserved diced apple, cover, and refrigerate the streusel.

Preheat the oven to 425°F (218°C). In a medium bowl, whisk together full fat sour cream, pure canned pumpkin, apple juice, granulated sugar, light brown sugar, egg yolk, and vanilla extract until smooth. Fold in diced apple.

In a separate bowl, whisk together all-purpose flour, baking soda, salt, and pumpkin pie spice. Gradually fold into pumpkin mixture using a spatula until no flour streaks remain.

Line a muffin tin with paper liners and fill cups about two-thirds full with muffin batter. Sprinkle chilled streusel topping over each muffin. Bake at 425°F for 5 minutes for initial rise.

Reduce oven temperature to 350°F (177°C) and continue baking for an additional 11 to 13 minutes. Check for doneness with a toothpick; it should come out clean or with a few moist crumbs.

Remove muffins from the oven and let cool in the pan for about 10 minutes. Transfer to a wire rack to cool completely.