Preheat the oven to 425 F (220 C). Line a standard 12-cup muffin pan with paper liners.
Add the ½ cup all-purpose flour, ⅓ cup light brown sugar, 1 tablespoon granulated sugar, ¾ teaspoon ground cinnamon, and ¼ teaspoon salt to a medium bowl and stir to combine. Add the 3 ½ tablespoons melted unsalted butter and stir until the mixture forms coarse, chunky crumbs. Refrigerate while you prepare the muffin batter. (Note 1)
Add the 2 ½ cups all-purpose flour, 2 teaspoons baking powder, ½ teaspoon baking soda, ½ teaspoon salt, 1 ½ teaspoons ground cinnamon, and ¼ teaspoon ground nutmeg to a medium bowl. Whisk until evenly combined and set aside.
Add the ½ cup softened unsalted butter, ½ cup granulated sugar, and ½ cup light brown sugar to a large mixing bowl. Beat with an electric mixer on medium speed until light and fluffy, about 2 to 3 minutes.
Beat in the 2 large eggs one at a time, mixing well after each addition. Add the 2 teaspoons vanilla extract mix just until combined.
Pour in the ¾ cup buttermilk and mix on low speed just until combined. The mixture may look slightly curdled at this point; this is normal. (Note 2)
Add the dry ingredients to the wet ingredients and fold gently until just a few streaks of flour remain. Add the diced apples and continue folding just until the apples are evenly distributed and no dry flour remains. (Note 3)
Divide the batter evenly among the 12 prepared muffin cups. The cups will be almost full.
Spoon about 1 heaping tablespoon of the chilled streusel over each muffin. Gently press the streusel into the batter with the back of a spoon to help it stay in place as the muffins bake. (Note 1)
Bake on the center rack at 425 F (220 C) for 5 minutes. Without opening the oven door, reduce the oven temperature to 375 F (190 C) and continue baking for about 18 minutes, or until a toothpick inserted into the center of a muffin comes out with a few moist crumbs. (Note 4)
Let the muffins cool in the pan for 5 minutes, then transfer them to a wire rack. If adding the maple icing, let the muffins cool for about 10 minutes before drizzling so the icing doesn't melt completely into the warm muffins.
Add the 1 cup powdered sugar, 2 tablespoons pure maple syrup, 2 teaspoons whole milk, ¼ teaspoon vanilla extract, and pinch of salt to a small bowl and whisk until smooth and thick but pourable. If the icing is too thick, add additional milk, 1 teaspoon at a time. If it is too thin, add powdered sugar, 1 tablespoon at a time. (Note 5)
Drizzle the maple icing over the muffins. Serve warm or at room temperature.