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Cake3 large eggs, separated\u00bd cup (4 ounces or 115 grams) unsalted butter, at room temperature1\u00bd cups (300 grams) granulated sugar1\u00bd teaspoons vanilla extract1\u00bd cups (375 grams) mashed bananas\u2154 cup (160 grams) sour cream3 cups (400 grams) all-purpose flour\u00bd teaspoon ground cinnamon1 teaspoon baking powder1\u00bd teaspoons baking soda1\u00bd teaspoons kosher salt
\nFilling and topping2 cups (12 ounces or 340 grams) semi- or bittersweet chocolate chips\u00bd cup (100 grams) granulated sugar1 teaspoon ground cinnamon
\nHeat oven: To 350\u00b0F (175\u00b0C). Coat a 9\u00d713-inch baking pan with nonstick spray and line the bottom with parchment paper, or if you have a larger sheet of parchment paper, you can use it to line the bottom and the sides, no spraying needed.
\nMake the batter: In a large bowl, beat egg whites until they hold firm peaks and set aside. [If you only have one mixer bowl, you can transfer the egg whites to another and use the mixer bowl to make the rest of the cake batter.]
\nIn an empty mixing bowl, beat butter and 1\u00bd cups of the granulated sugar until light and fluffy. Beat in egg yolks and vanilla. In a separate bowl, whisk flour, baking soda, baking powder, \u00bd teaspoon of the cinnamon, and salt together. Add banana to butter mixture and beat until combined. Add half the flour mixture, the sour cream, then the remaining flour mixture, beating each until combined. Fold in the egg whites.
\nAssemble the cake: Combine remaining \u00bd cup of the granulated sugar and 1 teaspoon cinnamon in a small bowl. Spread half of batter evenly in the prepared pan. Sprinkle with half of the cinnamon-sugar mixture and 1 cup of the chocolate chips. Dollop remaining cake batter over filling in spoonfuls. Use a rubber or offset spatula to gently spread it over the filling and smooth the top. Sprinkle batter with remaining cinnamon-sugar and remaining chocolate chips.
\nBake the cake: For 35 to 40 minutes, until a tester inserted into the center of the cake comes out clean. Let cool in the pan. [Note: This takes less time in my oven than the classic version does, which is usually 40 to 50 minutes.]
\nTo serve: Transfer cake to a cutting board and cut into squares.
\nDo ahead: My preference is to keep this cake at room temperature and not cover it. I will, however, press a piece of foil or plastic against the cut side. I don\u2019t like to cover the pan or put the cake in an airtight container because it softens the flaky cinnamon-sugar topping, which feels tragic.<\/p>\n