Preheat your oven to 400° F (200° C). Line a muffin tin with paper liners and set it aside so it's ready for the batter later.

In a bowl, whisk together 3/4 cup all-purpose flour, 1/4 cup packed light brown sugar, 2 tablespoons granulated sugar, and 1 teaspoon ground cinnamon (optional). Pour in 4 tablespoons of melted unsalted butter and mix until the mixture forms small clumps. Set the crumble aside for topping the muffins later. I like to use my fingers at the end to pinch the crumble into even chunkier pieces.

In a medium bowl, whisk together 2 cups all-purpose flour, 1 1/2 teaspoons baking powder, 1/2 teaspoon salt, 1 tablespoon ground cinnamon, and 1/2 teaspoon ground nutmeg (optional). Set this mixture aside.

In a large bowl, use an electric mixer to beat together 1/2 cup packed light brown sugar, 1/4 cup granulated sugar, and 1/2 cup unsalted butter (room temperature) until the mixture is light and fluffy. Add in 2 large eggs, 1/4 cup sour cream or Greek yogurt, and 1 teaspoon vanilla extract. Mix until well combined, scraping the sides and bottom of the bowl as needed.

Add half of the dry mixture from Step 3 into the wet ingredients from Step 4 and mix just until combined. Pour in 1/2 cup milk (room temperature) and mix again. Add the remaining dry ingredients and mix gently, being careful not to overmix. Finally, fold in 1 1/2 cups chopped Granny Smith or Honeycrisp apples by hand. I like to use tart apples for a nice contrast in flavor and texture.

Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full. Generously sprinkle the crumble mixture from Step 2 over each muffin, ensuring all the streusel is used.

Bake the muffins at 400° F (200° C) for about 20 minutes, or until a toothpick inserted into the center comes out with little to no crumbs. Let the muffins cool in the pan for a few minutes, then transfer them to a wire rack to cool completely. I like to enjoy these muffins slightly warm, when the topping is extra crisp.