Very soft variegated donut

Preparation
1. Prepare the basic dough
In a large bowl, beat the eggs and sugar using an electric mixer until the mixture is light and fluffy.
Add the oil and milk in a thin stream, continuing to mix.

Gradually incorporate the sifted flour with the baking powder, mixing with a spatula or a hand whisk.
Add the vanilla extract or lemon zest and mix well to obtain a smooth, lump-free dough.
2. Create the variegated effect

Divide the dough into two equal parts.
In one of the two bowls, add the sifted bitter cocoa and mix until completely incorporated.
Pour the two doughs (white and cocoa) alternately into a buttered and floured donut mold (diameter 24 cm), creating layers or pouring them in spoonfuls for a marbled effect.
Use a toothpick or fork to create spirals in the dough and obtain the variegated effect.