Preheat oven to 300° F (about 150° C).
Using an electric beater or kitchen aid, cream together sugar and butter, when it is light and fluffy, add egg yolks one at a time,beat well after each addition.
Blend in vanilla and yoghurt.
In a separate bowl, sift sifted flour, salt, and soda.
Add sifted dry ingredients to batter, beat well.
For the egg whites, to ensure a successful formation of stiff peaks, the first step is that the bowl - preferably a metal or glass bowl, and beaters should be clean and dry. If there is a trace of oil residue in the bowl or a little hint of yolk in the whites, it will interfere with the formation of air bubbles. Beat on low speed for about a minute until foamy then switch to medium/high speed until stiff peaks form. (Note: I do not have 2nd spare bowl designed for the kitchen aid for beating the egg whites, so I scoop with a spatula the yoghurt mixture batter - it would be rather stiff by then - into a large separate clean bowl; then wash, dry the beaters and bowl of the kitchen aid with paper towel.).
Take a quarter or so of the beaten egg white and mix it into the yoghurt batter, it will bring the consistency of the batter closer to that of the foam so they mix better. Fold the rest of the egg white in 2 or 3 parts, do not over mix.
Pour into a greased loaf pan and bake for about 1 1/2 hours (90 mins) or until loaf is golden brown and toothpick inserted in center comes out clean.
Cool in pan for 5 minutes, then transfer to wire rack and cool completely.