Preheat the oven to 180 degrees. Grease and flour a large bundt pan (12 cup).
Beat the eggs and sugar till creamy.
Add the oil and yogurt and beat till combined.
Sift in the flour and baking powder and mix with a whisk till no lumps.
Add the vanilla and mix finally.
Pour 3/4 of the batter into the prepared pan.
Into the remaining batter, fold in the cocoa powder.
Drop spoonfuls on the top and swirl with a fork.
Bake for 45 minutes till done. Cool in pan for 15 minutes and then flip onto to cooling rack. Cool completely.