Preheat the oven to 350 degrees. Line a cupcake pan with cupcake liners.
Beat the butter in a mixing bowl until fluffy. Add the egg whites, milk, key lime juice and zest, and oil. Mix well.
Combine the flour, sugar, baking powder, and salt in a different bowl. Whisk.
Add the dry ingredients to the wet ingredients and mix on low for 1 minute. Scrape the side of the bowl and mix on medium speed for another minute.
Add the cupcake batter to the liners, 2/3 full.
Put the pan in the oven and bake for 14-16 minutes.
Let the cupcakes cool before adding frosting.
To make the frosting, mix the butter and shortening. Add the key lime juice, zest, salt, and half the powdered sugar. Mix on low until most of the sugar is incorporated. Scrape the sides of the bowl and then add the rest of the powdered sugar.
Mix on medium speed for one minute. If you need to add more lime juice to thin out the frosting, you can.
Add the frosting to the pastry bag that has a large round tip on the end.
Once the cupcakes are cooled, pipe the frosting on top starting at the outer edge.
Add the crushed graham crackers by putting the crumbs on a plate and then carefully following the edges of the frosted cupcakes to coat them.
Slice the limes and add one in the middle of the frosting.