Blueberry Syrup
Combine cornstarch and water in a small dish and set aside.
In a medium saucepan bring blueberries, sugar, and water to a boil, stirring occasionally. Reduce heat and simmer for about 10 minutes, mashing the blueberries as they cook.
Add the cornstarch mixture and mix to combine, about 2 minutes. Turn off heat and set aside to cool.
Crumble
Whisk together flour, cinnamon, sugar, and a pinch of salt.
Add the butter, diced into cubes and work the butter into the flour mixture with your fingers until a crumbly mixture has formed. Set aside in the fridge.
Muffins
Preheat oven to 425F
In a medium bowl whisk together flour, cinnamon, baking powder and salt. Set aside.
In a large bowl, beat the butter and sugar until light and fluffy.
Add the eggs one at a time, beating well after each addition.
Add the vanilla and mashed bananas and mix until combined.
Add the dry ingredients to the creamed mixture alternatively with the milk. Mix gently after each addition until everything is just combined. Don't overmix.
Fold in the blueberries.
Line a muffin tin with liners and evenly scoop out batter.
Add blueberry syrup on top of muffin batter and sprinkle with crumble topping.
Bake at 425F for 5 minutes, then reduce to 350F for 15 mins.