Heat 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
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.