Preheat the 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 the buckwheat flour, white rice flour, corn starch, baking powder, and salt in a large bowl.
In another bowl beat the sugar with the oil. Add eggs, one at a time, beating well after each addition. Add the vanilla extract.
Pour the mixture into the dry ingredients along with the buttermilk. Mix well.
Add the cocoa powder and baking soda, and incorporate everything. Mix 3/4 cup of the chopped chocolate.
Pour the mixture into the prepared pan and place the remaining chopped chocolate on top.
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 clean, with no streaks of batter.
Remove from the oven and let cool for 10 minutes, then lift out by the parchment paper.