Pre-heat oven to 350 F (177 C).
In a boiling pot of water, cook sweet potato cubes for 15 - 20 minutes until fork tender.
Once cooked, add sweet potato to a food processor and blend until creamy and smooth.
Mix 2 tablespoons of ground flax with 6 tablespoons of water. Let sit for at least 5 minutes. This will create flax "egg" goo.
Melt butter.
In a bowl, combine melted butter, sugar and vanilla.
Mix in flax goo and sweet potato until well combined.
In a separate bowl, whisk flour, cocoa powder, cinnamon, baking powder and salt.
Add dry ingredients to wet ingredients slowly; I do a third at a time. Mix well.
Stir in chocolate chips and marshmallows.
Bake for 35 - 40 minutes.
This will not result in a clean toothpick even when it's fully cooked because it's so fudgy. Let cool and rest for several hours before serving.