Preheat the oven to 375 F (190 C) for 20 - 30 minutes before baking the muffins. Line 1 -2 (12) count muffins pans with cupcake liners, alternating cups.
In a large bowl, whisk together the flour, brown sugar, sugar and ground cinnamon for the crumble topping. Work in the melted butter a tablespoon at a time until a sand like texture develops with some bigger and smaller crumbs. Place the bowl in the fridge until the muffins are ready for topping.
½ cup (60 g) all purpose flour , ¼ cup (50 g) light brown sugar, 2 tablespoons (25 g) sugar, ½ teaspoon ground cinnamon , 3 - 4 tablespoons (57 g) melted butter
In a small bowl whisk the brown sugar, sugar, ground cinnamon and salt together until well combined. Set aside the cinnamon swirl mixture for now.
¼ cup (50 g) light brown sugar, ¼ cup (50 g) sugar, 2 teaspoons ground cinnamon , ¼ teaspoon salt
In a medium bowl whisk the all purpose flour, baking powder, baking soda and salt together until well combined. Set aside for now.
2 cups (240 g) all purpose flour , 2 teaspoons baking powder, ¼ teaspoon baking soda , ½ teaspoon salt
In a larger mixing bowl whisk the melted butter, light brown sugar, sugar, eggs, vanilla extract, sour cream and milk together until smooth and well combined. Stir in the the dry ingredients until just combined.
½ cup (113 g) unsalted butter, melted , ½ cup (100 g) light brown sugar , ¼ cup (50 g) sugar, 2 large eggs, room temperature, 2 teaspoons (10 ml) vanilla extract , ½ cup (120 g) sour cream, room temperature, ½ cup (120 ml) milk
Assemble the muffins. Scoop the vanilla muffin batter into each of the cupcake liners. Fill the cupcake liners about half way. Top the muffin batter with the cinnamon swirl. Top the cinnamon swirl with more muffin batter, followed by the crumble topping.
Bake the muffins for 20 -22 minutes or until a toothpick inserted into the center comes out clean or with a few moist crumbs. Cool the muffins in the pan for about 10 minutes. Remove the muffins from the pan to a cooling rack.
Top the coffee cake muffins with powdered sugar or with the vanilla glaze. To make the glaze whisk the powdered sugar with vanilla extract and milk until smooth and well combined.
1 cup (120 g) powdered sugar, ½ teaspoon vanilla extract , 2 - 3 tablespoons (30 - 45ml) milk or heavy cream