In a separate medium bowl, whisk the melted butter and oil together until smooth. Add the egg and vanilla extract, then mix in the sour cream. Whisk until fully combined and smooth.

Pour the wet ingredients into the dry mixture and gently fold them together using a spatula until just combined. Be careful not to overmix. Set the batter aside while you prepare the streusel topping.

Melt the butter and let it cool slightly.

In a medium bowl, whisk together the flour, sugars, cinnamon, and salt.

Drizzle the melted butter over the dry ingredients and use a fork to claw the mixture until it forms a crumbly texture with both large and small clumps.

Preheat the oven to 425°F (220°C). Line a 12-count muffin tin with paper liners. Rest the muffin batter for 15-30 minutes while the oven preheats.

Once the batter has rested, divide it evenly among the muffin tin cups. Top each muffin with a generous amount of streusel topping.

Place the muffin tin on the center rack of the preheated oven. Bake for 8 minutes, then without opening the oven door, reduce the temperature to 350°F (175°C) and continue baking for 12-14 more minutes, or until the tops are lightly golden brown and a toothpick inserted comes out clean or with a few moist crumbs.

Allow the muffins to cool in the tin for 7-10 minutes before carefully transferring them to a cooling rack.

If desired, whisk together powdered sugar and milk until smooth. Drizzle over the cooled muffins. Let the glaze set before serving.