Preheat oven to 425°F (218°C) for the initial high-heat bakery burst. Line a 12-cup muffin tin with tulip paper liners or grease thoroughly with nonstick baking spray.

Make the streusel: In a medium bowl, combine brown sugar, flour, rolled oats, and cinnamon. Cut in cold cubed butter using a pastry cutter or fingers until crumbly pea-sized clusters form. Set in the fridge while prepping the batter.

Prep the apples: In a small bowl, toss the finely diced peeled apples with 2 tablespoons sugar and 1 teaspoon cinnamon. Set aside.

Whisk dry ingredients: In a large bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg until well aerated.

Whisk wet ingredients: In a separate bowl, whisk melted butter, granulated sugar, brown sugar, eggs, buttermilk, and vanilla extract until glossy and fully combined.

Combine gently: Pour wet ingredients into dry ingredients. Fold with a rubber spatula just until barely moistened. Fold in the spiced apples and any accumulated juices. Do not overmix—lumpy batter is key to tender muffins!

Fill cups & top: Divide the thick batter evenly among the 12 muffin cups, filling them all the way to the top rim. Generously pile the chilled streusel topping over each muffin, gently pressing crumbs into the batter so they adhere.

The bakery-dome secret: Bake at 425°F (218°C) for exactly 5 minutes to trigger a rapid thermal burst of steam that lifts the muffins into high domed peaks.

Without opening the oven door, reduce oven temperature to 350°F (175°C) and bake for an additional 15 to 17 minutes until tops are golden brown and a toothpick inserted into the center comes out clean.

Cool in the pan for 10 minutes, then transfer to a wire cooling rack. If desired, whisk powdered sugar and milk together and drizzle over the warm muffins. Serve warm with whipped salted butter!