Preheat the oven to 350 degrees Fahrenheit and grease a 10-inch cast iron skillet (oven-safe) with vegan butter.
Begin by adding the flaxseed meal + water to a small bowl, stirring until combined and letting it sit for 5-10 minutes. Set aside.
In a large bowl, add the melted butter and sugar together, whisking until combined.
Add in the flaxseed meal mixture, pumpkin puree, milk, and vanilla, whisking to combine.
Now, add in flour, cocoa powder, pumpkin spice, salt, and baking powder, mixing everything together a spatula or wooden spoon, until fully combined. Fold in the cacao nibs. **NOTE: Batter will be thick, that's OK**
Pour the batter into the prepared skillet and evenly spread it in. Sprinkle the cacao nibs (or chopped chocolate) over the top of batter, lightly pressing them in.
Bake for 35-40 minutes or until the center comes out "mostly clean". During the last 10-15 minutes, add the vegan marshmallows on one side of the skillet brownie.
Once done, remove from oven and let cool for about 5-10 minutes before scooping a few spoonfuls of brownie and topping it with your favorite dairy-free ice cream.
Bon Appetit!