In a stand mixer or large bowl, beat the eggs with the sugar, lemon zest, and vanilla. Once the mixture is light and fluffy, add the milk and melted butter, and continue mixing.

Next, add the sifted flours and baking powder. The batter should be soft and able to form ribbons when lifted. If needed, add more flour.

Now, take half of the batter and mix it with the cocoa powder in a separate bowl. If it becomes too thick, add a few tablespoons of milk.

Pour the plain batter into a greased bundt pan, then pour the chocolate batter in the center. For a decorative effect, swirl with a toothpick if desired.