In a small mixing bowl, combine 3 tablespoons all-purpose flour, 1 1/2 tablespoons cold, cubed unsalted butter, 1 1/2 tablespoons light brown sugar, 2 teaspoons granulated sugar, a pinch of salt, and 1/4 teaspoon ground cinnamon or pumpkin pie spice. Using a fork or your fingers, cut the butter into the flour until the mixture resembles small pea-sized pieces—you'll get the best texture if you use your fingers for this small amount. Stir in 1 1/2 tablespoons finely diced apple. Refrigerate the streusel topping until ready to use.

Preheat your oven to 425°F (218°C) and adjust your oven rack to the middle position. Line a muffin pan with 4 muffin liners to prepare for baking.

In a medium mixing bowl, combine 1 1/2 tablespoons full-fat sour cream (at room temperature), 5 tablespoons pure canned pumpkin, 1 tablespoon apple juice (or orange juice or water), 3 tablespoons granulated sugar, 2 tablespoons light brown sugar, 1 large egg yolk (room temperature), and 1/2 teaspoon vanilla extract. Mix with a fork until smooth and well combined. Stir in 1/2 cup of apple chopped into small cubes.

In a small bowl, whisk together 1/2 cup plus 1 tablespoon all-purpose flour, 1/4 teaspoon baking soda, 1/8 teaspoon salt, and 3/4 teaspoon pumpkin pie spice. Sprinkle these dry ingredients over the wet mixture from Step 3. Using a spatula, gently fold the dry into the wet ingredients until just combined and no dry streaks remain—do not overmix to keep your muffins tender. I like to stop folding as soon as I see the last bit of flour disappear to avoid chewy muffins.

Evenly divide the muffin batter into the prepared muffin liners. Top each muffin with equal portions of the streusel topping (from Step 1). Bake at 425°F (218°C) for 5 minutes to help the muffin tops puff up. Then, without opening the oven, reduce the heat to 350°F (177°C) and continue baking for 11 to 13 minutes, or until a toothpick inserted in the center comes out mostly clean. I always rotate the pan halfway through baking to ensure even browning.

Let the muffins cool in the pan on a wire cooling rack for 10 minutes. Then carefully remove them from the pan and cool completely on the wire rack. Muffins can be stored covered at room temperature for 3 to 5 days.