Prepare the crumble: In a bowl, mix flour, cinnamon, salt, brown sugar, and walnuts. Stir in melted butter until crumbly. Chill in the fridge.
Preheat oven to 350°F (175°C). Grease and line an 8×8-inch pan with parchment paper.
In a bowl, whisk flour, baking powder, baking soda, and salt.
In another bowl, combine sugars and lemon zest, rubbing zest into sugar. Whisk in melted butter.
Add mashed banana, sour cream, eggs, and vanilla. Mix until smooth.
Fold dry ingredients into wet mixture gently. Stir until just combined.
Fold in blueberries carefully.
Pour batter into prepared pan.
Sprinkle crumble evenly on top.
Bake for 45–50 minutes until a toothpick comes out clean.
Cool before serving. Enjoy your Blueberry Banana Coffee Cake warm or at room temperature.