1. Preheat the oven to 350°F (176°C). Line the bottom of three 8-inch cake pans with parchment paper and grease the sides.
2. In a large mixing bowl, beat the butter and sugar together on medium speed until light in color and fluffy, 3-3. minutes.
4. Add the sour cream, vanilla extract and key lime zest and mix until combined.
5. Add the egg whites in two batches, mixing until incorporated after each addition. Scrape down the sides of the bowl as needed to make sure everything is combined.
6. Combine flour, baking powder and salt in a medium bowl. Combine the milk, water and key lime juice in a small measuring cup.
7. Add half of the flour mixture to the batter and mix until combined. Add the milk mixture to the batter and mix until combined. Add the remaining flour mixture and mix until smooth. Scrape down the sides of the bowl as needed to ensure everything is being combined.
8. Divide the batter evenly between the prepared cake pans. Bake for 22-24 minutes, or until a toothpick inserted in the middle comes out with a few crumbs.
9. Remove cakes from the oven and allow to cool for 3-4 minutes, then remove from pans to a cooling rack to finish cooling.
10. In a small bowl, combine the graham cracker crumbs, sugar and melted butter until crumbly, then spread onto a lined cookie sheet. Bake at 350°F (176°C) for 5 minutes, then remove and set aside to cool.
11. To make the frosting, beat butter and shortening together until smooth.
12. Add about half of the powdered sugar and mix until smooth.
13. Add keep lime juice and sweetened condensed milk and mix until smooth.
14. Add remaining powdered sugar and mix until smooth. Color the frosting with the yellow and leaf green icing colors. I used a very small amount of both. It won’t take much to get a nice lime green color.
15. When you are ready to build the cake, remove the domes from the cakes with a large serrated knife, so that they are flat.
17. Add the first layer of cake to a serving plate or a cardboard cake circle.
18. Top the cake with a layer of frosting. To get the frilly edges like I have, I used the Ateco 844 icing tip.
19. Top the frosting with about 1/3 of the graham cracker crumbs.
20. Add the second layer of cake and repeat steps 18 and 19 with the second layer of cake, then top the cake with the third layer of cake.
21. Spread about half of the remaining frosting onto the top of the cake, then pipe frosting around the edges. I used Ateco tip 808, a large round tip.
22. Sprinkle more graham cracker crumbs over the top of the cake and around the base of the cake, if desired.
23. Combine the additional sweetened condensed milk, powdered sugar and key lime juice in a small bowl, then use a spoon to place the drizzle around the outside edges of the cake. Finish off the cake with the key lime slices.