Preheat oven to 350°F (180°C).
Lightly grease the bottom and sides of a loaf pan or line with parchment paper. I used a loaf pan 9x5-inch. Set aside.
Whisk buckwheat flour, white rice flour, corn starch, baking powder, salt, and ground cinnamon in a medium bowl. Set aside.
In another bowl beat the sugar with the oil. Add eggs, one at a time, beating well after each addition. Add the vanilla extract.
Mash the bananas with a fork and add to the preparation.
Add the dry ingredients along with the milk. Mix well until all the ingredients are incorporated. Add walnuts if desired.
Pour the mixture into the prepared pan.
Bake for 50 minutes or until cooked, keep in mind that all ovens are different. To know if it's ready insert a toothpick into the center of the cake, it should come out with no streaks of batter.
Remove from the oven and let cool for 10 minutes, then lift out by the parchment paper.
When completely cooled spread dulce de leche on the surface and sprinkle with shredded coconut.