In a small bowl, combine the oat milk and lemon juice. Let sit for 5-10 minutes to make your "buttermilk"
Combine the flour, baking soda, baking powder, and salt in a medium bowl.
Melt the vegan butter and whisk with 1 cup of sugar in a large bowl.
Add the "buttermilk" mixture and vanilla extract.
Fold in the dry ingredients until the mixture is almost fully incorporated
Toss in the blueberries and gently fold to combine.
Cover and chill for 1-12 hours
Preheat the oven to 375℉
Add the blueberry muffin bread dough to a greased bread loaf pan (or divide equally among 12 greased muffin tins)
Top with a generous sprinkle of demerara sugar before baking
Bake for 5 minutes at 375℉, then reduce the temperature to 350℉ and bake for another 25 minutes (or for another 15 minutes, if making muffins)
Cool completely in the pan before serving. Enjoy!