Prepare the Cinnamon Sugar Layer: Begin by melting the butter. Mix in the brown sugar and cinnamon until it forms a thick consistency. Set this mixture aside to thicken while you prepare the muffin batter.
Mix the Dry Ingredients: In a medium bowl, combine the flour, granulated sugar, baking powder, salt, cinnamon, and nutmeg. Stir well to integrate the ingredients and set the bowl aside.
Blend the Wet Ingredients: In a separate, larger mixing bowl, whisk together the eggs, buttermilk, vegetable oil, and vanilla extract for about 30 seconds until thoroughly combined.
Combine the Batter: Gradually fold the dry ingredients into the wet mixture by hand, mixing gently until just combined. The batter should be thick. Cover the bowl with a towel and let it rest for 15 minutes. Meanwhile, preheat the oven to 425°F (220°C).
Prepare the Crumb Topping: While the batter rests, create the crumb topping. In a bowl, mix together the flour, brown sugar, cinnamon, salt, and melted butter. Stir with a spatula until the mixture forms a sandy texture. Use your hands to press the mixture together into clumps and set aside.
Fill the Muffin Pan: Line a 12-cup muffin pan, but only fill 6 of the wells to allow room for the muffins to expand. Spoon 2 tablespoons of batter into each lined muffin cup. Add 1 teaspoon of the cinnamon sugar mixture to the center of each muffin and spread it gently. Cover with an additional 4-6 tablespoons of batter, filling each cup completely.
Top the Muffins: Sprinkle the crumb topping over each muffin, pressing it gently to adhere. For an optional touch, add a sprinkling of sparkling sugar.
Bake the Muffins: Place the pan in the center of the oven. Bake at 425°F (220°C) for 7 minutes. Then, without opening the oven door, reduce the temperature to 350°F (175°C) and continue baking for 16-20 minutes or until a toothpick inserted into the center comes out clean.
Cool the Muffins: Allow the muffins to cool in the pan for 15 minutes before removing them. Repeat the process with any remaining batter, ensuring the oven temperature is reset to 425°F.
Prepare the Glaze (Optional): In a small bowl, mix together the powdered sugar, vanilla extract, and milk. If the glaze is too thin, add more powdered sugar, a teaspoon at a time. Drizzle over the cooled muffins if desired.