Prepare the Streusel Topping by combining all-purpose flour, unsalted cold butter, granulated sugar, brown sugar, salt, and ground cinnamon in a bowl. Mix until the texture resembles small clumps, then fold in diced apples and refrigerate.

Preheat your oven to 425°F (220°C) and line a muffin pan with muffin liners, ready for four muffins.

In a medium mixing bowl, blend the wet ingredients: sour cream, pumpkin, apple juice, light brown sugar, granulated sugar, egg yolk, and vanilla. Whisk until smooth and fold in the diced apples.

In another bowl, whisk together flour, baking soda, salt, and optional pumpkin pie spice. Ensure an even mixture.

Fold the dry mixture into the wet mixture carefully, aiming for a tender crumb. Stop mixing as soon as combined.

Distribute the muffin batter into the liners, filling about two-thirds full, and top generously with the streusel mixture.

Bake at 425°F for 5 minutes, then reduce the temperature to 350°F (175°C) and bake for another 11-13 minutes, or until a toothpick comes out clean.

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