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 buckwheat flour, white rice flour, corn starch, baking powder, and salt in a large bowl. Set aside.
In another bowl beat the eggs with the sugar, oil, and vanilla extract.
Add the wet ingredients to the dry ones, along with the milk. Integrate everything.
Pour 3/4 of the preparation into the pan. Mix the cocoa powder with the remaining 1/4 of the preparation and integrate. Dump on the pan, over the vanilla preparation. Mix with a spoon to achieve marbling.
Cook 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 loaf cake, it should come out clean, with no streaks of batter.
Remove from the oven and let it cool for 10 minutes, then lift out by the parchment paper.
Sprinkle the cake with powdered sugar.