Preheat oven to 350°F (175°C). Grease and flour a Bundt pan.
In a large bowl, beat the eggs and sugar until light and fluffy.
Mix in melted butter, vanilla extract, and milk/yogurt.
In a separate bowl, whisk together flour, baking powder, and salt.
Gradually combine dry ingredients with wet ingredients, stirring until smooth.
Separate 1/3 of the batter and mix it with cocoa powder for the marble effect.
Pour the vanilla batter into the Bundt pan, then add the cocoa batter on top. Swirl gently with a knife.
Bake for 40-45 minutes, or until a toothpick inserted in the center comes out clean.
Let cool before removing from the pan. Dust with powdered sugar or drizzle with melted chocolate.
Slice and enjoy with coffee or tea!