Preheat oven to 425 degrees F. Line a baking sheet with parchment paper.
If you haven't, boil sweet potatoes, let cool, mash it. You need 1 1/2 cups to make this recipe.
In a small bowl, add together the milk and vinegar. This makes vegan buttermilk. Let sit together for 5 minutes. Then, add in brown sugar, melted butter, cinnamon, vanilla and nutmeg. Combine well, set aside.
In a separate large bowl, add flour, baking powder, and salt. Whisk together. Grate in your butter to the bowl. After about ever 4 tbsp, fold the butter in with flour (this helps it not clump and stay cold). Stir together one last time to make sure well combined.
Add in sweet potato mixture, careful not to overmix. The dough will be a bit rough and still have bits here and there.
Onto a lightly floured surface, dump out the bowl. Use yours hands to gather the dough together and press into a rectangle. The dough should be scraggly/crumbly. Press the dough down to about 1 1/4 inches thick.
Fold the top 1/3 of the dough into the middle. Then, fold the bottom 1/3 to the middle. (Similar to how you would fold a paper. Flour dough a little if sticky. Press the dough out to 1 inch thick and repeating folding process.
Trim off the rough edges to square off the sides to help with the rising. Then cut into 8 equal portions. Transfer to prepared baking sheet, spacing biscuits evenly.
In a small bowl, combine the topping ingredients together. Then brush the tops lightly with the mixture (you should only use half of it). Bake for 20-25 minutes. When done, brush again evenly with remaining cinnamon butter mixture.
Serve immediately or let cool and store in an airtight container.