Preheat the oven to 350℉. Line a 12-count muffin tin with liners or grease with non-stick cooking spray.
Whisk together the oil, white sugar, brown sugar, eggs, and vanilla in a large mixing bowl. Add the buttermilk and whisk until combined.
In another mixing bowl, whisk together the flour, baking powder, and salt. Stir in the blueberries until coated with flour. Add to the wet ingredients and stir just until no streaks of flour remain. Scoop the batter into the prepared muffin tins, filling each well to the top with batter*.
Stir together the melted butter, brown sugar, flour, and chopped pecans to form a crumbly streusel. Top the unbaked muffins with the prepared streusel, distributing evenly between all the muffins.
Bake muffins in the preheated oven for 23-27 minutes or until a toothpick inserted into the center comes out clean. Let cool for about 10 minutes in the pans before transferring to a wire rack to finish cooling. Enjoy!
Notes
Use any neutral-flavored cooking oil such as vegetable oil. I often use avocado oil.
If you don't have buttermilk, place 1 tablespoon of white vinegar in the bottom of a measuring cup and pour milk on top to the ¾ cup measurement. The milk will curdle and thicken, use in place of real buttermilk.
Or use walnuts or almonds. You may also completely omit the nuts.
*This creates large muffin tops. If you would like more and smaller muffins, fill each well about ½-3/4 full. Store fully cooled muffins in an airtight container that has been lined with paper towels for up to 4 days. However, the streusel topping will be the most fresh and crunchy on the day they were baked.