Prepare the cinnamon sugar layer by melting the butter, stirring in the brown sugar and cinnamon until thick, and setting it aside to thicken while making the muffin batter.

In a mixing bowl, combine the flour, granulated sugar, baking powder, salt, cinnamon, and nutmeg. Stir to combine and set aside.

In a larger mixing bowl, combine the eggs, buttermilk, vegetable oil, and vanilla extract. Stir to combine, then gradually mix in the dry ingredients until just combined. The batter will be thick.

Allow the batter to rest for 15 minutes and preheat the oven to 425ºF.

During the 15-minute rest, make the crumb topping by combining flour, brown sugar, cinnamon, salt, and melted butter. Mix well until the mixture looks wet and crumbly.

For bakery style muffin tops, line a muffin pan with liners and fill every other well with batter. Add cinnamon sugar mixture, cover with more batter, and sprinkle crumb topping over each muffin.

Bake at 425ºF for 7 minutes, then reduce the temperature to 350ºF and bake for 16-20 minutes or until a toothpick inserted in the center comes out clean.

Allow the muffins to cool in the pan before removing. Repeat with the remaining batter.

For the glaze (optional), combine powdered sugar, vanilla extract, and milk in a bowl. Drizzle over the muffins.