In a small bowl, mix together streusel topping. Start by mixing sugar, flour, and cinnamon. Add in the cold pieces of butter and work into flour with finger tips until it is crumbly. Add in chocolate chips and mix. Place in freezer until ready to use.
Preheat oven to 425F. Grease a jumbo muffin tin (6 muffins).
In a large bowl, mash bananas. Add in sugars, sour cream, oil, vanilla, and eggs. Whisk until fully combined.
Add in flours, flaxseed, cinnamon, baking powder, baking soda, and salt. Whisk together again.
Add in flour coated blueberries, stir to combine.
Pour batter into muffin tins, filling almost to the top of each muffin hole.
Top with streusel topping and lightly pat into batter.
Bake at 425F for 10 minutes. Then reduce heat to 350F and bake for another 23-25 minutes until toothpick inserted comes out dry.
Allow to cool in the pan for 5 minutes before moving to a rack to cool.