Preheat your oven to 375°F. Line a 12 count muffin tin with paper liners, and set aside.
Make the streusel topping: In a medium bowl, combine the streusel ingredients. Mash together with a fork until you're left with a dry, crumbly mixture. Set aside to prep the muffin batter.
Make the blueberry muffin batter: In a medium bowl, whisk together your flour, baking powder, cornstarch, and salt. Set aside.
In a large bowl, combine your sugar, milk, oil, eggs, and vanilla. Whisk to combine.
Pour your dry ingredients into your wet ingredients. Mix together until just combined. (Some clumps are totally normal!) Fold in your blueberries until just combined as well.
Portion your muffin batter into the prepared muffin tin, filling each cup to the very top (for an extra tall muffin top!) Evenly sprinkle about 1 to 2 tablespoons of streusel on top of each muffin.
Bake the muffins in the center of your preheated 375°F oven for 20 to 25 minutes, or until golden brown on top, and a skewer inserted into the center of a muffin comes out clean of any batter. Cool slightly before serving!