Streusel: 

1. Make the streusel: In a small mixing bowl, whisk together the flour, granulated sugar, brown sugar, cinnamon, and salt. Use a spatula to stir in the melted butter until fully incorporated, and then work the mixture into a single clump of dough. Transfer to the freezer to solidify while making the muffins.

Muffins:

1. Preheat oven to 375ºF and light grease a 12-cup muffin tin with nonstick spray. Use a paper towel to wipe off any spray that may have settled on the top surface of the pan.
2. Whisk dry ingredients: In a medium mixing bowl, whisk together the flour, baking powder, and salt. Set aside.
3. Mix wet ingredients: In a large mixing bowl, whisk together the granulated sugar, brown sugar, and melted coconut oil until well-combined.Add the eggs and beat in until well-incorporated; the mixture should get more pale in color as you whisk. Add the milk and vanilla and whisk in until fully incorporated. Stir in the mashed banana just to combine.
4. Combine: Fold the dry ingredients into the wet just until mostly incorporated. The batter will be thick. Add the chocolate chips and fold in just until evenly dispersed, with no streaks of flour remaining in the batter. Be careful not to overmix.
5. Transfer to muffin pan: Divide the muffin batter evenly between the prepared muffin cups. Sprinkle a few extra chocolate chips onto the top of each. Remove the streusel mixture from the freezer and use your hands to crumble into pieces somewhere between the size of a pea and a small blueberry. Sprinkle the crumb topping over the muffins.
6. Bake and cool: Bake muffins in preheated oven for 20-25 minutes, until the crumb topping is just starting to turn golden in spots and a toothpick inserted into the center of one of the muffins comes out clean, or with just a few moist crumbs. Allow the muffins to cool in the pan for about 10 minutes before carefully transferring to a wire rack to cool completely (be cautious to avoid breaking off the streusel crumbs). Cool to room temperature before adding the icing drizzle.

Icing Drizzle: 

1. Make the icing: In a small bowl, whisk together the confectioner's sugar, milk, and vanilla extract until smooth. Add more milk if needed to achieve desired consistency - you're looking for a thick icing that will hold its shape when drizzled.
2. Drizzle over muffins: Use a piping bag fitted with a small round tip, a parchment piping bag with a narrow opening, or a spoon to drizzle the icing over the muffins. Let set.