- 2 1/4 cups all-purpose flour
- 1 teaspoon baking soda
- 1/2 teaspoon salt
- 1 stick unsalted butter, softened, plus 2 tablespoons, melted and cooled
- 1 cup sugar, divided
- 2 large eggs
- 1 1/4 cups mashed very ripe bananas (about 3 medium)
- 2/3 cup plain whole-milk yogurt
- 1 teaspoon pure vanilla extract
- 1 (3 1/2- to 4-ounce) bar 70%-cacao bittersweet chocolate, coarsely chopped
- 1 cup walnuts (3 ounces), toasted , cooled, and coarsely chopped
- 1/2 teaspoon cinnamon
- Preheat oven to 375°F with rack in middle. Butter a 9-inch square cake pan.
- Stir together flour, baking soda, and salt.
- Beat together softened butter (1 stick) and 3/4 cup sugar in a medium bowl with an electric mixer at medium speed until pale and fluffy, then beat in eggs 1 at a time until blended. Beat in bananas, yogurt, and vanilla (mixture will look curdled).
- With mixer at low speed, add flour mixture and mix until just incorporated.
- Toss together chocolate, nuts, cinnamon, melted butter, and remaining 1/4 cup sugar in a small bowl. Spread half of banana batter in cake pan and sprinkle with half of chocolate mixture. Spread remaining batter evenly over filling and sprinkle remaining chocolate mixture on top.
- Bake until cake is golden and a wooden pick inserted in center of cake comes out clean, 35 to 40 minutes. Cool cake in pan on a rack 30 minutes, then turn out onto rack and cool completely, right side up.
Cooks’ note:Cake can be made 2 days ahead and kept in an airtight container at room temperature.