Preheat oven to 210°C / 410°F.
Prepare a 12 muffins pan with paper cups or grease it with cooking non-stick spray
In a large bowl, sift the flour together with the baking powder, salt and bicarbonate of soda. Mix briefly.
In another container add the eggs, sugar, vanilla, buttermilk and oil, mix until everything is well integrated. (notes 1)
Transfer the liquid ingredients to the bowl with the dry ingredients and mix (or beat with a hand whisk) just until everything is moistened and lump-free. Do not overmix.
Divide the batter into two equal parts separated into two containers.
On one of the bowls, sift the cocoa powder and mix until you obtain the chocolate batter.
Now the vanilla and chocolate mixtures are transferred to the muffin tin, alternating both flavors to get the marble effect. Fill each muffin cup to the top.
Bake in the preheated oven for about15 minutes or until a skewer inserted into the center of one of the muffins comes out clean.
When you take them out of the oven, let them rest for 5 minutes in the mold and then unmold them on a wire rack.
Let them cool completely and they are ready to enjoy.