Preheat the oven to 400°F and line a muffin tin with liners.
If making streusel, whisk the dry streusel ingredients together in a small bowl (white sugar, light brown sugar, ground cinnamon, and flour). Then use a fork to mash the butter in until combined.
In a large mixing bowl, whisk together the melted vegan butter, white sugar, apple cider vinegar, vegan milk and vanilla extract until smooth.
Add in the flour, baking powder, baking soda and salt and mix to combine. Be careful not to over-mix!
Fold in your sliced blackberries. If you are using fresh blackberries then they might break apart slightly so try to be gentle while adding them to the muffin batter.
Fill the muffin liners to the top and generously sprinkle over your streusel topping. Bake for 12-16 minutes, a toothpick inserted in the middle of a muffin should come out clean. Cool for 5 minutes in the muffin tin before transferring them to a wire rack to cool completely.
Video
Notes
Please Note: I highly recommend using a kitchen scale to weigh your dry ingredients. Metric measurements are found above under the "Metric" tab.Flour: Be sure to properly measure your flour using the scoop and level method or a kitchen scale. Too much flour will make the muffins dry and crumbly.Optional Step - Chilling the Muffin Batter: For taller, more domed muffin tops, chill the muffin batter in the mixing bowl before baking. You can skip this step and the muffins will still taste delicious, so it's optional.Storage and Freezing Instructions: Store muffins for 2-3 days at room temperature in an airtight container.You can freeze the cooled muffins in a ziploc freezer bag for up to 2-3 months. When ready to eat, take one out of the freezer and microwave it for 30 seconds, or let it thaw at room temperature.