Preheat oven to 180ºC . In a bowl, combine flour, sugar, baking powder, baking soda, and 0.125 tsp. salt. Add buttermilk, shortening (butter or margarine) and vanilla. Beat on low speed for 30 seconds, scraping bowl. Beat on medium to high speed for 2 minutes, scraping bowl occasionally. ADd egg whites and beat for 2 minutes more, scraping bowl. Pour into 2 greased and floured 9 x 1 cm round baking pans. Bake for 30-35 minutes or until a toothpick comes out clean. Cool on wire racks for 10 minutes. Loosen sides and remove from pans. Serves 12. NOTE: One day my husband Pat wanted a marble cake. So, I improvised. To make a marble cake from this recipe, simply reserve one third of the cake batter before putting in the cake pans. Then, melt one square unsweetened or semi-sweet chocolate either over boiling water or in the microwave oven. Stir to blend. Add to the cake mixture and mix well. Put one half of the chocolate batter in each cake pan in dollops, and swirl carefully, so as not to mix the white batter with the chocolate. What you are trying to achieve in a marble effect. Bake as directed. (BTW, this is GREAT!!)