Preheat oven to 375F. In a medium-size bowl beat the egg yolks with the sugar for about 2-3 minutes or until pale yellow. Add the butter cut in small pieces, and beat until incorporated. In a separate bowl, beat the egg whites until stiff.
In a small bowl mix the flour, baking powder and salt until combined. Add this to the egg yolk mixture slowly, alternating with the beaten egg whites, mixing after each addition. Suggested order: egg whites - flour - egg whites - flour - egg whites - flour. Add vanilla and lemon and orange zest, and mix until just combined. Do not overmix!
Pour the mixture into a baking pan lined with parchment paper. Bake for about 35 minutes or until top of cake is golden brown. You can check for doneness by inserting a toothpick in the center of the cake - the cake is done if the toothpick comes out clean. Take the cake out of the oven and let it cool down completely on a rack. Cut into squares and serve decorated with buttercream frosting and fresh fruit on the side, if desired.