Preheat your oven to 425°F (220°C). Line a 12-cup standard muffin tin with parchment paper liners or grease generously. Starting at high heat creates that signature tall bakery dome!

In a small bowl, fork together 1/2 cup flour, 1/3 cup brown sugar, 1 tsp cinnamon, and 3 tbsp melted butter until coarse, crumbly clusters form. Set aside in the refrigerator. In another small bowl, toss the finely diced apples with 1 tbsp sugar and 1/2 tsp cinnamon.

In a large bowl, whisk together 2 cups flour, baking powder, baking soda, salt, 1 tsp cinnamon, and nutmeg. In a medium bowl, whisk melted butter, granulated sugar, brown sugar, eggs, buttermilk, and vanilla until smooth and emulsified.

Pour the wet ingredients into the dry ingredients. Fold gently with a spatula just until barely combined (a few flour streaks are fine; do not overmix!). Gently fold in the spiced diced apples.

Divide batter evenly among the 12 muffin cups, filling each all the way to the top rim. Press a generous tablespoon of cinnamon streusel crumble over each muffin crown, gently patting so it adheres.

Bake at 425°F (220°C) for 5 minutes to trigger a rapid oven-spring rise. Without opening the oven door, reduce the temperature to 350°F (175°C) and bake for an additional 15 to 17 minutes until tops are golden and a toothpick inserted in the center comes out clean. Cool in the pan for 5 minutes, then transfer to a wire rack to serve warm!