Preheat oven to 170 ° Celsius. Grease bottom and sides of 4 , 6 -inch round cake tins and line bottom with parchment paper.( I have 2, I baked 2 cakes first and reused them to bake the other two cakes.)
In the bowl of stand mixer beat butter and sugar until light and fluffy about 5 minutes.
Sift together flour, salt and baking powder, set aside.
Add whole eggs one after the other beating well after each addition and then egg white. Add vanilla extract and beat until combined.
With mixer on low, alternately add the flour mixture and buttermilk, beginning and ending with flour mixture.
Divide batter evenly between cake tin. (I used a measuring cup and a scale).
Bake for 20 - 25 minutes or until tooth pick inserted in centre comes out clean. Leave to cool tin on wire racks for 10 minutes, then invert onto rack gently, discard parchment paper and cool completely.
Wrap cakes in cling film and chill in the fridge for 1- 3 hours.