Preheat the oven to 350F/177C. Line 10 large muffins tins or 12 to 14 cupcake tins with paper, or spray the inside with baking spray. You can also butter and flour the tins.
In a large mixing bowl, combine all the wet ingredients: buttermilk, oil, eggs, and vanilla, plus the sugar. Beat the ingredients together for 2 to 3 minutes on medium speed until the eggs are well-beaten.
In a separate bowl, combine the dry ingredients: flour, baking soda and powder, salt and ground cinnamon. Sift the dry ingredients into the muffin batter, then beat on medium speed for about 1 minute, just until the dry ingredients are well incorporated. Watch my video recipe to see how it’s done.
In a small bowl, toss together the blueberries with the cornstarch until the berries are well coated. (If using frozen berries, thaw them first) Fold the berries into the muffin batter. Fill each lined muffin or cupcake tin about three-quarters of the way full. Top each muffin with extra blueberries, if desired.
Making the Cinnamon Topping:
Next, prepare the cinnamon crumble topping. In a medium bowl, combine the flour, brown sugar, cinnamon and nutmeg. Toss the ingredients together until they’re well combined. Next, pour in the melted butter. Using your fingers, work the melted butter into the dry ingredients, until fine crumbs form and no dry flour is remaining. Spoon a few tablespoons of crumble over each muffing. If you don’t like too much crumble, cut the recipe in half.
Baking the Muffins:
Bake the muffins in the preheated oven at 350F/177C for about 20 minutes, or until a toothpick inserted into the center comes out clean (about 17 minutes for cupcake tins). Let the muffins cool slightly before enjoying.
For storing these blueberry muffins, allow them to cool completely, then place into an air-tight container and store at room temperature.