1. Preheat oven to 350F (180C).
2. Grease a 9-inch cake pan and line the bottom with parchment paper. You can use two 8-inch round cake pans if you like and share the batter equally. Put it aside.
3. In a medium bowl combine flour, baking powder, baking soda and salt.
4. Beat the butter with an electric mixer for about 30 seconds. Add in sugar, 1 whole egg and vanilla extract. Beat until combined well.
5. Add egg whites and beat for about a minute.
6. Add plain yogurt and mix well.
7. Stir n the flour mixture. Stir with a spatula until combined.
8. Pour the batter into the prepared cake pan and bake for 40-45 minutes or until a toothpick inserted in the center comes out clean.
9. Let it cool for 10 minutes and remove from the pan. Let it cool completely. Better if chilled.
10. Slice the cake horizantally using a cake slicer or a knife.

For the frosting:

1. Maket he frosting right before you start icing the cake.
2. Mix butter in a large bowl with an electric mixer until creamy.
3. Add in the powdered sugar gradually. Add in vanilla and milk. Mix until smooth.
4. Add 1 or 2 tablespoons more milk if you think the frosting is too thick.
5. Place the bottom layer on a stand and spread 1 cup of icing on it. Gently place the top layer and spread some frosting over it. Try to spread it evenly on the sides until the entire cake is thinly coated. Chill for about 30 minutes.
6. Remove from the refrigerator and coat the entire cake with the rest of the frosting. Chill at least for 2 hours before serving.