1 Prepare the Streusel
In a small bowl, combine brown sugar, flour, and cinnamon
Cut in the cold butter using a fork or pastry cutter until crumbly
Set aside

2 Mix the Muffin Batter
Preheat oven to 375°F (190°C) and line a muffin tin with paper liners
In a large bowl, whisk together flour, granulated sugar, brown sugar, salt, and baking powder
In another bowl, mix vegetable oil, egg, milk, and vanilla extract
Pour the wet ingredients into the dry and stir until just combined (do not overmix)

3 Assemble the Muffins
Spoon batter evenly into prepared muffin cups, filling each about 2/3 full
Sprinkle streusel topping generously over each muffin

4 Bake and Cool
Bake for 18–22 minutes, or until a toothpick inserted in the center comes out clean
Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely

5 Add Glaze (Optional)
In a small bowl, whisk powdered sugar, milk, and vanilla extract until smooth
Drizzle over cooled muffins