Prepare the Streusel: In a medium bowl, combine all-purpose flour, cold cubed unsalted butter, light brown sugar, granulated sugar, a pinch of salt, and ground cinnamon. Mix until the mixture resembles coarse crumbs, with some pea-sized chunks of butter remaining. Fold in a few reserved diced apples, cover, and chill.

Prep the Muffin Batter: Preheat your oven to 425°F (220°C) and line a muffin pan. In a large bowl, mix together full-fat sour cream, pure canned pumpkin, apple juice, both sugars, egg yolk, and vanilla extract until creamy and well combined.

Combine Dry and Wet Ingredients: In a separate bowl, whisk together all-purpose flour, baking soda, salt, and pumpkin pie spice. Gradually add dry ingredients to the wet mixture, folding gently until just combined.

Fill the Muffin Liners: Evenly divide the batter among the prepared muffin liners, filling each about two-thirds full. Generously sprinkle the streusel topping over each muffin.

Bake to Perfection: Bake at 425°F (220°C) for 5 minutes, then reduce temperature to 350°F (175°C) and bake for an additional 11–13 minutes until golden brown.

Cool and Enjoy: Remove muffins from the oven, cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.