Preheat oven to 350°F. Fit a muffin tin with paper muffin cups or spray with cooking spray, set aside.
In a large mixing bowl, stir together the butter, sugar, mashed bananas, egg, vanilla extract, and milk until well-combined. Set aside.
In a second bowl, stir together the baking powder, baking soda, salt, and flour. Add the dry ingredients to the wet, and stir until just combined. Do not overmix! Fold in the chocolate chips, if using.
Spoon the batter into the prepared muffin tin, filling each cup up about ⅔ full. Set aside.
To make the streusel, mix together the brown sugar, flour, cinnamon, hemp seeds, and salt in a small bowl. Stream in the melted butter and stir until well-distributed. Sprinkle the streusel over top of each muffin, pressing in slightly to "stick."
Bake in preheated oven for 15-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Notes
This recipe will make 12 large muffins (think: coffee shop style) or 18 regular-sized muffins. You can also spoon the batter into a mini muffin tin and get 36—just reduce the baking time down to 10 minutes or so.