Preheat oven to 165°Celsius.

In a bowl, cream together butter and sugar until light and fluffy.

Add in egg yolk and vanilla, mixing until fully incorporated and set aside.

In a separate bowl, combine flour and baking powder.

Gradually add dry ingredients to the butter mixture, alternating with buttermilk. (a little buttermilk-mix-a little flour-mix-repeat) Mix until batter is fluffy, but don't overmix! (substitute buttermilk by adding half a tablespoon of lemon juice or vinegar to half a cup of milk and keep it aside for 10 minutes or till it curdles)

Beat egg whites until cloudy and thick (firm peaks). Very gently fold whites into the batter and mix just until incorporated.

Pour the batter into a greased and lightly floured/ baking paper lined tin. I used a 6.5 x 2.5 " springform tin.

Bake for 30-35 minutes or until a toothpick inserted comes clean. The top should spring back when touched. Let the cake cool in the pan until the pan is warm to touch, then remove from the pan and allow to cool completely. Frost with your favorite icing or eat as it is !