Preheat the oven to 350°F (175°C).
Grease a large Bundt pan with butter or cake release spray.
In a medium bowl, sift together the flour, baking soda, cocoa, salt and cinnamon (if using). Set aside.
Cream the softened butter and sugar in the large bowl of an electric mixer until light and fluffy.
Add the eggs, one at a time, beating after each addition.
Add the bananas, sour cream and vanilla and beat into the batter.
Remove the bowl from the electric mixer and stir in the flour mixture using a wooden spoon.
Pour the boiling water over the batter and stir in carefully.
Pop the bowl back on the electric mixer and beat the batter for about one minute.
Pour the batter into the Bundt pan.
Bake for 40 to 50 minutes, or until a skewer inserted in the centre of the cake comes out clean.
(If using layer cake pans the layers will take about 20 to 25 minutes to bake, Cupcakes will take 15 to 20 minutes to bake. The cakes are done when a skewer inserted in the centre comes out clean.)
Cool in the pan for 10 minutes before removing to a wire rack to cool completely.
When cool, dust with confectioner's sugar (icing sugar), glaze or frost with chocolate buttercream. Or just serve plain, slightly warm from the oven with some ice cream melting slowly over the top.