Batter:
3 cups (375 grams) unbleached cake flour1 tablespoon (15 grams) baking powder1 1/2 teaspoons (5 grams) kosher salt1 1/2 cups (360 grams) mashed ripe banana (3 to 4 medium bananas)1 1/4 cups (275 grams) firmly packed light1 cup (240 grams) sour cream1/2 cup (113 grams) unsalted butter, melted (110°F/43°C to 115°F/46°C)1/3 cup (75 grams) neutral oil2 large eggs (100 grams), room temperature1 tablespoon (13 grams) vanilla extract
Topping:
1/4 cup (57 grams) unsalted butter, softened (61°F/16°C to 65°F/18°C)6 tablespoons (72 grams) granulated sugar2/3 cup (83 grams) all-purpose flour1 1/2 teaspoons (3 grams) ground cinnamon1/2 teaspoon vanilla extract1/4 teaspoon kosher salt
Filling:
1/2 cup (110 grams) firmly packed light brown sugar1 teaspoon ground cinnamon
For batter: In a medium bowl, whisk together cake flour, baking powder, and salt.
In a large bowl, whisk together banana, brown sugar, sour cream, melted butter, oil, eggs, and vanilla. Gradually stir in flour mixture just until combined. Cover and refrigerate for at least 1 hour or up to overnight.
For topping: In another medium bowl, stir together butter and granulated sugar until combined. Stir in flour, cinnamon, vanilla, and salt until well combined and mixture is crumbly. Cover and refrigerate until ready to use.
Preheat oven to 400°F (200°C). Line a 12-cup muffin pan with tulip-style paper liners.
For filling: In a small bowl, whisk together brown sugar and cinnamon.
Using a 3-tablespoon spring-loaded scoop, scoop batter into prepared muffin cups (about 45 grams each). Sprinkle 2 teaspoons (10 grams) filling onto batter in each cup, leaving a slight border around edges of batter. Using a 1⁄4-cup spring- loaded scoop, scoop batter (about 64 grams) onto filling in each cup; gently spread batter smooth with a small offset spatula. Sprinkle topping onto batter in small clumps.
Bake for 5 minutes. Reduce oven temperature to 375°F (190°C), and bake until a wooden pick inserted in center comes out clean and an instant-read thermometer inserted in center registers 205°F (96°C), 15 to 18 minutes more. Let cool in pan for 10 minutes. Serve warm, or remove from pan, and let cool completely on a wire rack. Store in an airtight container for up to 3 days.
