1. In a bowl, combine 1/2 cup flour, 1/4 cup brown sugar, 2 tbsp sugar, and 1/8 tsp kosher salt. Add 1/4 cup cold, diced butter and use your fingers to pinch and rub until the mixture forms pea-sized clumps. Refrigerate streusel topping.

2. Preheat oven to 425°F and line a 12-cup muffin tin with liners.

3. In a large bowl, whisk together 2 1/2 cups flour, 1 cup sugar, 2 tsp baking powder, 1/2 tsp baking soda, and 1/2 tsp kosher salt. Add blackberries and toss gently to coat.

4. In a separate bowl, microwave 1/4 cup butter until melted. Add bananas and mash well. Whisk in oil, Greek yogurt, eggs, and vanilla bean paste until smooth.

5. Pour wet ingredients into dry ingredients. Gently fold together with a spatula until just combined and no flour streaks remain; batter will be thick and lumpy.

6. Divide batter evenly among muffin cups, filling to the top.

7. Sprinkle chilled streusel generously over each muffin.

8. Bake at 425°F for 7 minutes. Lower temperature to 350°F (do not open oven) and bake for 13 more minutes, or until a toothpick inserted in the center comes out clean.

9. Cool muffins in the tin for 5-10 minutes, then transfer to a wire rack to cool completely.