In a medium bowl, mix together the all-purpose flour, unsalted butter, light brown sugar, granulated sugar, salt, and ground cinnamon using your fingers or a pastry cutter until the mixture resembles pea-sized crumbs. Gently fold in the diced apples, then cover the bowl with plastic wrap and refrigerate the streusel while you prepare the muffins.

Preheat your oven to 425°F (220°C) and line a muffin tin with liners. In a large mixing bowl, combine the full-fat sour cream, canned pumpkin, apple juice, sugars, egg yolk, and vanilla extract. Whisk this mixture until it’s smooth and creamy, ensuring all ingredients are well integrated before gently folding in the larger apple cubes for added texture.

In a separate bowl, whisk together the remaining all-purpose flour, baking soda, salt, and ground cinnamon. Gradually add this dry mixture into the wet pumpkin and apple batter, folding gently until just combined. Be careful not to overmix; a few lumps are perfectly fine.

Carefully spoon the muffin batter into the prepared muffin liners, filling each about two-thirds full for the best rise. Once all muffin cups are filled, generously sprinkle the chilled streusel topping over each muffin.

Place the muffin tin in your preheated oven and bake at 425°F (220°C) for 5 minutes. After this initial baking, reduce the heat to 350°F (175°C) and continue to bake for an additional 11-13 minutes, or until a toothpick inserted into the center of a muffin comes out clean.

Once baked, remove the muffin tin from the oven and allow the muffins to cool in the pan for about 10 minutes. Then, gently transfer the muffins to a wire rack to cool completely.