Prep the pan and oven: Heat oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease well.
Make the streusel: In a bowl, mix 3/4 cup flour, 1/3 cup brown sugar, 1 teaspoon cinnamon, and a pinch of salt.
Stir in 6 tablespoons melted butter until clumpy. Chill while you make the batter.
Whisk dry ingredients: In a large bowl, whisk 2 cups flour, 2 teaspoons baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon salt, 1 teaspoon cinnamon, and a pinch of nutmeg.
Mix wet ingredients: In a separate bowl, whisk 2 large eggs, 3/4 cup granulated sugar, 1/2 cup neutral oil, 3/4 cup milk, and 2 teaspoons vanilla.
Combine: Pour wet ingredients into dry. Stir gently until just combined.
A few small lumps are fine. Fold in the diced apples.
Fill the cups: Divide batter evenly among the muffin cups, almost to the top for taller muffins.
Add streusel: Break the chilled streusel into chunky crumbs and mound on top of each muffin. Press lightly so it adheres.
Bake: Bake 18–22 minutes, until the tops are golden and a toothpick comes out clean or with a few moist crumbs.
Cool: Let muffins rest in the pan 5 minutes, then transfer to a rack.
For extra shine, brush warm tops with a little melted butter and dust with cinnamon sugar (optional).