1 cup cocoa
3/4 cup boiling water
3/4 cup buttermilk
1/2 cup mayonnaise
1 1/3 cups light brown sugar
1 egg plus 1 egg white, at room temperature
1/2 teaspoon vanilla extract
1/2 cup pecans, toasted
1/2 cup all-purpose flour
1 cup plus 2 tablespoons cake flour
3/4 teaspoon baking powder
1/2 teaspoon baking soda
Caramel Pecan Topping
1/2 cup granulated sugar
1 tablespoon corn syrup
1/2 cup water
3 tablespoons cream
2 tablespoons butter
1 1/2 cups pecans, toasted and finely chopped
4 or 5 firm-ripe bananas
1/3 cup granulated sugar
Preheat oven to 350 degrees F. Lightly grease and flour a 10-inch springform pan.
To make the cake: Whisk together the cocoa and boiling water until smooth. Cover with plastic wrap and let cool to room temperature. Whisk in half of the buttermilk, mixing until smooth.
Combine the mayonnaise, brown sugar, egg, egg white and vanilla. Beat with an electric mixer until very light and nearly doubled in volume, about 5 minutes.
Combine the pecans, the 2 flours, baking powder, baking soda and the salt in a food processor and pulse until the mixture achieves a powdery consistency. Add to the beaten egg mixture in thirds, alternating with the remaining buttermilk. Fold in the chocolate mixture until just combined.
Pour the batter into the prepared pan and bake in the lower third of the oven for 25 to 30 minutes, until a cake tester inserted in the center comes out clean. Place on a rack and let cool to room temperature. (The cake may be made up to 8 hours in advance to this point)
To make the topping: Combine the sugar, corn syrup and water in a small, heavy-bottom saucepan. Place over medium heat and cook until the mixture turns amber, washing down any sugar crystals clinging to the sides of the pan using a pastry brush dipped in water. Remove from heat and slowly add the cream. If lumps form, return to heat briefly, stirring constantly, until smooth. Whisk in the butter and fold in the pecans.
The filling may be made up to 1 week in advance. Cover tightly and keep refrigerated. Reheat in a double boiler or microwave until quite warm (but not hot) and softened. Mixture will remain stiff.
To assemble: Dot the top of the cake with the caramel, then spread over the cake. This will be textured, do not expect a smooth topping.
Just before serving, preheat the broiler.
Cut the bananas diagonally into thin slices, discarding ends. Pat slices to flatten, then arrange on the cake in concentric circles, starting from the perimeter. Try to cover the top as completely as possible. Sprinkle with sugar and broil 3 to 5 inches from the heat source, until the sugar melts and turns golden. Watch carefully so it doesn't burn. Remove the rim of the springform pan and transfer the cake to a plate and serve immediately, topped with whipped cream or ice cream if desired.