Preheat the oven to 350°F and grease a 9x9 inch baking pan.
In a medium bowl, whisk together cake flour, baking powder, baking soda, and salt.
In a separate large bowl, cream together softened butter and granulated sugar until light and fluffy.
Add egg whites, vanilla extract, and lemon zest to the butter mixture, and beat until smooth.
Gradually add the dry ingredients and buttermilk to the wet ingredients, mixing until just combined.
Pour the batter into the prepared pan and bake for 36-42 minutes, or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 30 minutes, then transfer to a wire rack to cool completely.