Chocolate Banana Bundt Cake

8 oz. (2 sticks) unsalted butter
1/2 cup natural cocoa powder 

1 tablespoon instant espresso powder

1 cup water
About 2 1/2 cups all-purpose flour (10 ounces or 238g)

2 cups dark brown sugar (15 ounces; 426g)
1 teaspoon baking powder
1 teaspoon baking soda
1 teaspoon Diamond Crystal kosher salt; for table salt, use half as much by volume
1 1/2 cups coarsely mashed ripe bananas from 3 medium bananas (12 ounces or 340g)
1/2 cup sour cream, at room temperature

3 large eggs, at room temperature, lightly beaten
2 teaspoons pure vanilla extract
1/2 cup walnuts, chopped
1 tablespoon confectioners' sugar

Adjust oven rack to middle position and preheat oven to 350°F. Grease Bundt pan with baking spray.   In a medium saucepan, combine butter, cocoa powder, espresso powder, and water. Bring to a boil over medium-high heat, stirring constantly, until mixture is combined, about 2 minutes.

In a large bowl, whisk flour, brown sugar, baking powder, baking soda, and salt to combine. Whisk in cocoa mixture, followed by bananas, sour cream, eggs, and vanilla, until smooth.  Fold in walnuts.

Scrape batter into prepared pan and bake until tester inserted in center of cake comes out clean, about 45 minutes.  Transfer cake to cooling rack and cool 10 minutes, then invert directly onto rack and cool to room temperature, about 1 hour. Sprinkle with confectioners’ sugar using a fine-mesh strainer and serve.