Preheat oven to 350°F.
In a mixing bowl, beat butter, sugar and lemon rind using electric beater on high speed until creamy.
Now add egg yolks to the mixture and beat again.
In another bowl, combine yogurt and milk.
After that add yogurt mixture and mix in the batter.
Gradually add flour and mix using rubber spatula.
Beat egg whites separately Using electric beater until soft peaks would form.
Now slowly mix egg whites into the batter
Pour the batter into the greased pan and bake for 45 mins. After that reduce the temperature to 300 and bake for 10 mins more.
Cool the bread to room temperature before serving.