Heat the oven to 375°F. Line a standard 12-well muffin pan with paper liners or coat the wells with cooking spray.

Stir 4 tablespoons melted and cooled unsalted butter, ½ cup all-purpose flour, ¼ cup granulated sugar, ¼ cup packed brown sugar, 1 teaspoon ground cinnamon, and ¼ teaspoon kosher salt together in a medium bowl with a fork until evenly moistened and a variety of crumb sizes form. Do not mix into a paste of uniform consistency.

Whisk 2 cups all-purpose flour, 2 teaspoons baking powder, and 2 teaspoons ground cinnamon together in a medium bowl.

Whisk 4 tablespoons melted and cooled unsalted butter, ¾ cup granulated sugar, ¼ cup neutral oil, and ½ teaspoon kosher salt together in a large bowl. Whisk in ½ cup whole milk, 2 large eggs, and 1 tablespoon vanilla extract until combined.

Fold in the flour mixture and 1 peeled and diced medium apple with a flexible spatula until just combined. Do not overmix.

Divide the batter between the muffin wells (about generous ¼ cup per well). Break up the topping and sprinkle the crumb topping evenly over the batter (about 2 tablespoons per well).

Bake until the muffins are golden-brown and a toothpick inserted into the center of a muffin comes out clean, 22 to 25 minutes. Let cool for 5 minutes. Transfer the muffins to a wire rack and let cool completely, about 30 minutes.

Whisk ½ cup powdered sugar and 1 tablespoon milk together in a small bowl until smooth. If the glaze is too thick to drizzle, whisk in more milk a few drops at a time as needed. Drizzle the glaze evenly over the muffins with a small spoon and let sit until set, about 15 minutes.