Preheat the oven to 350°F (180°C). Grease an 8x8-sized dish and line it with parchment paper. Melt the two portions of vegan butter in separate dishes. Let them cool. Take the vegan yogurt and plant milk out of the refrigerator so they can start to come to room temperature.
For the streusel topping, combine the brown sugar, flour, and cinnamon in a small bowl. Stir in the melted vegan butter until it's crumbly. Don’t over-stir so that some of the crumbs remain larger. Cover the bowl and refrigerate it for now.
Rinse the blueberries and pat them dry. Coat them with 1/2 of a tablespoon of flour in a small bowl and set it aside. Mix the apple cider vinegar into the plant milk to create a vegan buttermilk. Set it aside for a few minutes to allow time for it to curdle.
Whisk the dry cake ingredients together (flour, cane sugar, baking powder, and cinnamon) in a bowl. In a separate bowl, combine the wet ingredients (vegan buttermilk, melted vegan butter, vegan yogurt, and vanilla extract).
Fold the wet and dry ingredients together just until they’re combined. Try not to overmix the batter. Then, fold in the blueberries.
Pour the batter into the baking dish. Stir the streusel topping and spread it over the batter. Bake it for 55-60 minutes. Test the center with a toothpick that should come out clean. Let it cool for at least 15 minutes.