Preheat your oven to 400 °F and line your muffin tin with liners.
Stir together the milk and apple cider vinegar in a small bowl or cup and let sit for 5 minutes.
While the milk mixture sits, stir together the flour, sugar, baking powder, baking soda, and salt.
Then stir the milk mixture, vegetable oil, and vanilla in with the dry ingredients to make the batter. It will be slightly thick.
Gently fold in the blackberries.
Scoop about ¼ cup of the batter into each muffin liner in your muffin tin (3/4 full).
Make the topping by stirring the brown sugar, flour, and cinnamon together. Add the melted butter and lightly mix with a fork just until the ingredients are crumbly.
Once done, remove from the oven and let cool before enjoying!
Top each muffin with the crumble topping.
Bake for about 20 minutes or when a toothpick insterted into the center of the muffin comes back with just a few crumbs. Cool on a cooling rack.
Notes
Store lightly covered at room temperature for up to 3-4 days for best results!
This recipe was calculated using the exact brands and measurements I used to make this recipe. If you are following a strict diet please note changing anything will cause the nutritional info to change. My calculations are intended as a guide only.