1. To make topping, combine all ingredients in a small bowl, making sure to incorporate butter cubes evenly. Set aside.
2. Preheat oven to 325 degrees. Lightly grease an 8-by-8-inch baking pan and line it with parchment paper.
3, In a small bowl, toss together thawed frozen blueberries,raspberries, and 1/4 cup all-purpose flour (to prevent juice from bleeding). Set aside.
4. In a medium bowl, whisk together remaining 2 1/4 cups all-purpose flour, baking soda, baking powder, and sea salt.
5. In a large bowl, whisk eggs and sugar together until well blended and creamy.
6. In a separate bowl, blend mashed banana with melted butter. (For ease of mashing, microwave banana and butter mixture together for 10 to 15 seconds.) Whisk until light and fluffy, making sure no banana chunks remain. Mix into egg and sugar mixture. Add vanilla, buttermilk, lemon zest, and lemon juice. Gradually add flour mixture to wet ingredients, stirring to incorporate after each addition.
7. Gently fold in blueberries and raspberries. Pour batter into prepared pan. Bake for 35 to 45 minutes. Remove cake from oven and add crumble with remaining 1 tablespoon lemon zest, spreading topping evenly over cake. Bake for another 15 minutes until a toothpick inserted into center of cake comes out clean. Remove from oven and let cool slightly, then remove from pan and finish cooling on a rack. Enjoy! xx Candice