Preheat oven to 350º F. Line a 12-muffin tin with paper liners.
In a large bowl, combine the buckwheat flour, almond flour, arrowroot flour, baking powder, baking soda, and sea salt. Stir to mix well.
Mash ripe banana until smooth.
In a large mixing bowl, combine the mashed banana, eggs, coconut oil, and vanilla. Whisk until smooth.
Add the dry ingredients into the bowl with the wet ingredients. Mix until no dry ingredients remain.
Fold in the blueberries (or any other additions!).
Divide batter between 12 paper muffin cups.
If using, combine the streusel ingredients in a small bowl and stir to combine.
Top the muffins with the streusel topping.
Bake muffins for 20 minutes. Allow the muffins to cool for about 15 minutes prior to removing them from the muffin pan.