Bakery-style banana blackberry muffins with tall, domed tops and a crisp oat streusel. Soft and fluffy with juicy bursts of blackberry, balanced sweetness, and a buttery crunch.
Optional quick-ripen: If bananas aren’t overripe, mix 1 1/3 cups mashed ripe banana with 2 large eggs in a large bowl, cover, and let sit 45–90 minutes.
Preheat oven to 425°F (220°C). Line a 12-cup muffin pan with baking cups.
Make streusel: In a small bowl, combine 2 tablespoons all-purpose flour, 1 tablespoon packed dark brown sugar, 1 tablespoon quick oats, and 2 teaspoons melted butter. Mix with a fork until crumbly; set aside.
Wet ingredients: In a large bowl, combine 1 1/3 cups mashed banana and 2 large eggs (or the quick-ripened mixture). Whisk in 2/3 cup packed dark brown sugar, 6 tablespoons melted butter, 1/2 cup plain Greek yogurt, and 2 teaspoons vanilla extract until smooth.
Dry ingredients: In a separate bowl, whisk together 1 1/2 cups all-purpose flour, 1/2 cup quick oats, 1 1/2 teaspoons cinnamon, 1 teaspoon baking powder, 1 teaspoon baking soda, and 1/2 teaspoon fine sea salt.
Combine: Add dry ingredients to wet and stir just until combined (do not overmix).
Scoop: Add about 2 tablespoons plain batter to each muffin cup. Gently stir 1 cup blackberries into remaining batter once, then divide to fill cups heaping. Top with streusel.
Bake at 425°F for 5 minutes. Without opening the oven, reduce to 350°F (177°C) and bake about 15 minutes more, until golden and a toothpick comes out clean.
Cool in pan 10 minutes, then transfer to a wire rack to cool completely.