Preheat the oven to 425°F. Line a 12-cup muffin tin with paper liners then spray with nonstick spray. Set aside.
Crumb topping: combine the sugars, cinnamon and salt in a medium bowl. Whisk in melted butter then stir in the flour using a rubber spatula. Set aside while you make the muffin batter.
1/3 cup granulated sugar, 1/3 cup brown sugar, 1 teaspoon cinnamon, 1/4 teaspoon salt, 1/2 cup unsalted butter, 1 1/2 cups all purpose flour
Dry ingredients: in a medium bowl, combine flour, brown sugar, baking powder, baking soda, cinnamon, and salt.
1 1/2 cups all purpose flour, 1/2 cup brown sugar, 2 teaspoon baking powder, 1 teaspoon cinnamon, 1/4 teaspoon baking soda, 1/4 teaspoon salt
Wet ingredients: In a large bowl, whisk together the milk, melted butter, vanilla and eggs.
3/4 cup milk, 1/3 cup unsalted butter, melted, 2 teaspoons vanilla, 2 large eggs
Finish the batter: Add the dry ingredients and stir with a rubber spatula until just incorporated, being careful not to over-mix. Let the muffin batter rest for 10 minutes. This activates the baking powder and develops the gluten for tall muffin tops.
Scoop and bake: In a small bowl, combine the brown sugar and cinnamon for the cinnamon swirl. Carefully scoop half the batter into the prepared muffin pan. Sprinkle the cinnamon swirl in the middle of each then top with the remaining batter almost all the way to the top.
1/2 cup brown sugar, 2 teaspoons cinnamon
Generously top each with the reserved crumb topping, using your fingers to gently press the crumbs into the batter to help it stay in place while baking. It will be a lot of crumb topping on each muffin. Bake muffins in preheated oven for 7 minutes, then reduce the heat to 350°F degrees and bake another 16-18 minutes, until set and a toothpick inserted in the center comes out with a few moist crumbs
Icing: Combine powdered sugar and milk in a medium bowl and whisk until smooth. If the glaze is too thin, add more confectioners' sugar as needed.
1/4 cup powdered sugar, 2-3 teaspoons milk
When the muffins are done, cool for 10 minutes and drizzle the glaze on each muffin using a small spoon.