Preheat oven to 350 degrees F (175 degrees C). Line muffin pan with paper cupcake liners.
Whisk together cake flour, baking powder and salt.
In a separate bowl, whisk together egg whites and buttermilk.
Place the sugar in your mixer bowl and add the lemon zest. Rub the zest into the sugar with your fingers until moist and fragrant. Add the butter to the sugar mixture and beat at medium speed for 3 minutes, until very light.
With mixer on low, add one third of the flour mixture, then add half of the buttermilk mixture. Add half of the remaining dry ingredients, then the rest of the buttermilk mixture, and finally the remaining dry ingredients. Scrape down the bowl then beat on medium-high for 2 minutes to ensure it is thoroughly mixed and well aerated.
Spoon batter into prepared muffin cups, filling them about 2/3 full.
Place a round marble in between the cupcake liner and the pan. This will create an indent into the cupcake liner, causing the cake to bake into a heart shape.
Bake in the preheated oven until cupcakes are very lightly browned and a toothpick inserted into the center of a cupcake comes out clean, 20 to 25 minutes.
Cool cupcakes in the pans for 5 minutes; transfer cupcakes to cooling rack to finish cooling. Be careful, the marbles will be hot!
Separate the frosting into three bowls. Keep one white and tint the other two using the pink and red gel food coloring.
Frost the cupcakes, accentuating the heart shapes by spreading the frosting over the lip of the liner for the heart bumps, and down into a point for the bottom of the heart. Decorate with sprinkles, sugar, nonpareils etc.
I've provided you with my favorite white cake recipe here, but this can be done just as easily with a box mix.
If you would like your cupcakes to look even more like hearts - simply drag the frosting down at the bottom into a point.
Adults love these fun valentine cupcakes just as much as the kids and make a great edible gift to your sweetheart.
Gel food coloring is best. A little goes a long way and the colors are a lot more vibrant. Liquid food coloring can change the consistency of the frosting because you have to add so much more to achieve the color you want.