To a medium-size bowl, add all-purpose flour, cornstarch, baking powder, baking soda, and a pinch of sea salt, and mix with a whisk.
To a mixing bowl, add white granulated sugar, sunflower oil, vegan yogurt, and lemon zest. Whisk until well combined. Next, add the vegan buttermilk and mix until combined.
To the wet mixture, sift dry ingredients, and mix until just combined. Don’t overmix the batter. Next, fold in fresh blueberries.
Evenly divide the batter into 12 muffin liners. Crumble the streusel on the top and place the pan into the preheated oven.
Bake at 400°F (200°C) for 5 min, then turn the oven to 350°F (180°C) and bake for 15-20 min. The muffins are done when a toothpick inserted into the middle comes out clean. If it looks wet, bake for another 5 minutes because ovens can vary.
Let the muffins cool in the pan for about 15-20 minutes, then add them to a cooling rack and let cool completely.