Whisk the egg and sugar in a bowl until foamy.
Add oil, vanilla extract, orange peels, and soda, then mix the ingredients.
Add the buckwheat flour and baking soda by sifting.
Grease the cake mold with butter and then flour it.
Pour half of the mixture into a small cake mold, add cocoa to the rest and mix them.
Pour the cake mix with cocoa on the cake mold and mix gently with a fork (mix the colors).
Bake in a preheated 170-degree oven for 40 minutes. ( do not open in the first 20 min.)