Start by preparing the muffin batter. In a large mixing bowl, whisk together the flour, both sugars, cornstarch, baking powder, cinnamon, baking soda, and salt until evenly combined.
1 ¾ cups all-purpose flour, ½ cup light brown sugar, ½ cup granulated sugar, 2 tablespoons cornstarch, 2 teaspoons baking powder, 1 teaspoon ground cinnamon, ½ teaspoon baking soda, ¾ teaspoon sea salt
In a separate bowl, whisk together the melted butter and oil until smooth. Add the egg and vanilla extract, followed by the sour cream, and whisk until fully combined and creamy.
6 tablespoons unsalted butter, ⅓ cup vegetable oil, 1 large egg, 1 ½ teaspoons vanilla extract, 1 cup sour cream
Pour the wet ingredients into the dry ingredients and gently fold together using a spatula until just combined. Be careful not to overmix- stop as soon as no streaks of flour remain. Set the batter aside to rest for 15-30 minutes.
While the batter is resting, preheat your oven to 425°F (220°C) and line a 12-count muffin tin with paper liners.
To make the streusel topping, cut the butter into pieces and melt it, then set it aside to cool slightly. In a bowl, mix together the flour, sugars, and cinnamon. Once the butter is no longer hot, drizzle it over the dry ingredients and use a fork to gently combine. The mixture should be crumbly with a mix of small and larger clumps-avoid overmixing into a paste.
5 tablespoons salted butter, 1 cup all-purpose flour, ⅓ cup light brown sugar, ⅓ cup granulated sugar, 1 teaspoon ground cinnamon
Once the batter has rested, divide it evenly between the muffin cups, filling each about ¾ full.
Sprinkle the streusel topping generously over each muffin.
Place the muffins in the oven and bake at 425°F (220°C) for 8 minutes. Without opening the oven door, reduce the temperature to 350°F (175°C) and continue baking for another 12–14 minutes, or until the tops are lightly golden and a toothpick inserted into the center comes out clean or with a few moist crumbs.
Allow the muffins to cool in the tin for 10 minutes, then carefully transfer them to a wire rack to cool completely.
For the glaze, whisk together the powdered sugar, vanilla extract, and milk until smooth and pourable.