Line a 12 cup muffin tray with muffin liners and set it aside. Preheat the oven to 425 F/220 C.
In a small bowl, combine the brown sugar, cinnamon, and melted butter until combined and crumbs form. Chill in the freezer until ready to use.
1 cup dark brown sugar, 1 tablespoon cinnamon, 3 tablespoon unsalted butter
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
3 ¼ cups all-purpose flour, 2 ½ teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon salt
In a separate large mixing bowl, whisk together the buttermilk, melted butter, sugar, egg, sour cream, oil, and vanilla.
1 ½ cups buttermilk, ½ cup unsalted butter, 1 cup granulated sugar, 2 large whole eggs, ¼ cup sour cream, ¼ cup canola oil, 1 tablespoon vanilla extract
Whisk the dry ingredients into the wet ingredients until almost combined, with some flour streaks still remaining.
Reserve ½ cup of the cinnamon crumb for the tops of the muffins. Fold the rest of the crumbs into the batter until just combined.
Scoop about ⅓ cup (110 g) of the batter into each cup of the lined muffin tray. The batter should fill higher than the muffin liner. (see photo in the post for reference)
Evenly sprinkle the rest of the crumb topping on the tops of each muffin.
Bake at 425 for 8 minutes, then decrease the heat to 350F /180 C and continue baking for another 14-18 minutes, or until a toothpick inserted into the center comes out clean.
Once cool, combine the powdered sugar and softened cream cheese until smooth.
⅓ cup powdered sugar, 3 ounces cream cheese
Whisk in the milk and vanilla extract until the glaze is thick but pourable.
¾ teaspoon vanilla extract, 2-3 tablespoon milk
Drizzle the cream cheese glaze over the tops of the muffins and enjoy!