In a small bowl, combine the brown sugar, granulated sugar, cinnamon, salt, and flour. Use a fork or your fingertips to cut in the cold, cubed butter until the mixture forms pea-sized crumbs. Place the bowl in the fridge while you make the muffin batter. Keeping it cold helps the topping bake up crisp and crumbly.

In a large mixing bowl, whisk together the flour, granulated sugar, brown sugar, baking powder, baking soda, salt, and cinnamon. Make sure everything is well distributed to avoid any lumps of leavening.

In a medium bowl or large measuring jug, whisk the oil, eggs, buttermilk, and vanilla extract until fully combined and smooth.

Pour the wet mixture into the bowl of dry ingredients. Use a rubber spatula or wooden spoon to gently fold everything together. Mix just until no streaks of flour remain—don’t overmix or the muffins will become dense.

Divide the batter evenly among the 12 muffin cups. Fill each one about 3/4 full. The batter will rise quite a bit.

Remove the chilled streusel from the fridge and generously spoon it over the top of each muffin. Press it down lightly so it sticks to the batter, but don’t pack it in too tightly.

Bake for 18–22 minutes, or until the muffins are golden on top and a toothpick inserted in the center comes out clean or with just a few crumbs.

Cool the muffins in the tin for 5 minutes, then transfer them to a wire rack to cool completely. This helps the bottoms stay soft and prevents overbaking.