To make the vegan "buttermilk" add the soy milk and apple cider vinegar to a medium bowl, stir and allow to sit for 5 minutes.
Add the buttermilk, pumpkin puree, almond butter, maple syrup, and vanilla extract to a medium bowl and whisk to combine.
To a separate large bowl, add the brown sugar, flour, baking powder, baking soda, milled flaxseeds, sea salt, and pumpkin spice mix. Whisk to combine.
Add the wet ingredients to the bowl of dry ingredients and whisk to combine, be careful not to overmix.
Using an ice cream scoop or large spoon, distribute the batter amongst the muffin liners. Crumble some streusel topping on top of each muffin.
Bake for 22-24 minutes. Check they are done by piercing them with a toothpick or a knife, the knife should come out almost clean- a few crumbs are fine. Allow the muffins to cool for 5 minutes before transferring them to a cooling rack to cool further for 30 minutes.