Prepare the cinnamon sugar layer by melting the butter, then stir in the brown sugar and cinnamon until thick. Set aside to thicken.

In a mixing bowl, combine the all purpose 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 for 30 seconds.

Gradually mix in the dry ingredients, mixing by hand just until combined. Cover the bowl with a towel and allow the batter to rest for 15 minutes. Preheat the oven to 425?F during this time.

Meanwhile, make the crumb topping by combining all purpose flour, brown sugar, cinnamon, salt, and melted butter in a mixing bowl. Mix until it resembles wet sand.

Line a 12 count muffin pan with 6 muffin liners, filling every other well with batter.

Fill the liners with 2 tablespoons of batter. Drop 1 teaspoon of the cinnamon sugar mixture in the center of each muffin, spreading it with fingers. Then add 4-6 tablespoons of batter, covering the cinnamon sugar.

Crumble the crumb topping over each muffin, pressing it on slightly. Optionally, sprinkle with sanding sugar.

Place one pan in the center of the oven and bake for 7 minutes at 425?F. Then, turn the temperature down to 350?F and bake for an additional 16-20 minutes, or until a toothpick inserted in the center comes out clean. Allow to cool in the pan for 15 minutes before removing. Repeat with remaining batter, ensuring to bump the temperature back up to 425?F if needed.

(Optional) For the glaze, combine powdered sugar, vanilla extract, and milk in a small mixing bowl and drizzle over muffins if desired.