In a medium bowl, whisk together the flour, baking powder and salt. Set aside.

In the bowl of a stand-up mixer with the paddle attachment (alternatively you can do this in a large bowl with an electric hand mixer), add the sugar and key lime zest. Beat the two together. This will release all of the fragrant oils in the lime zest and infuse them in the sugar. We want this, we need this.

Add the butter and vanilla extract; beat until light and fluffy, about 2 to 3 minutes. Add in the oil, vanilla extract; and beat until smooth.

If you want the cake to be a “key lime color”, add a very small amount of yellow and green food coloring gel. This is of course optional.

In a measuring cup, pour in the milk, egg whites, and add the strained key lime juice.

Turn the mixer to low speed and, alternating between the dry ingredients and the liquid ingredients, add them to the butter/sugar mixture, until the batter is relatively smooth.

Pour the batter evenly into the three prepared baking pans (about 470 grams of batter per pan) and smooth it out until it reaches the sides.

Transfer the cakes to the oven to bake for 22 to 25 minutes, until a skewer inserted into the center comes out clean.

Allow the cakes to cool in the pan for about 10 minutes and then invert onto a cooling rack and cool to room temperature. It’ll probably take a good hour for them to cool completely.

Place the pot over medium low heat and continue whisking until the mixture thickens about 5 minutes. Then remove the pot from the heat and whisk in the butter. Strain the key lime curd into a small bowl, cover the surface with plastic wrap and refrigerate for at least 30 minutes or until completely chilled.

In the bowl of a stand-up mixer with the paddle attachment (or a medium bowl using an electric hand mixer), add the cream cheese and beat it until smooth. I like to do this or else the cream cheese frosting will have little clumps of cream cheese. Next, add the butter and beat it until smooth, about 15 seconds.

Add the powdered sugar, salt, vanilla and heavy cream. Start on slow speed so the powdered sugar doesn't fly everywhere. And then gradually turn it to medium speed. Beat for a full 2 minutes to ensure fluffy, delicious frosting.

In two small bowls, add about 2 tablespoons of frosting to each. You can eyeball this. One will be our key limes (lighter green) and another will be our stems (green).

If your frosting is super soft, add about 1 tablespoon of powdered sugar to each bowl and mix it in. This will help stiffen it up.

Add a drop or two of yellow food coloring to one frosting bowl, along with one drop of green food coloring. This will be our key limes. In the second container, add a drop or two of green food coloring gel to the second frosting bowl. Mix until smooth. Transfer the lighter green frosting to a piping bag fitted with a small round piping tip. And do the same with the second frosting.

Place one cooled cake layer on a cake board. Add about 1/3 cup of white frosting to the top of the first layer and smooth out into a thin even layer. Place the second cake layer on top. Repeat with the third layer. Add a thin crumb coat of white frosting to the cake and smooth the top and sides. Transfer the cake to the fridge to chill for about 20 minutes. Then add a second layer of white frosting to the cake.

To add the key limes, pipe on rounds of frosting; they’ll end up looking like key limes once we add the tops. Add as many as you like. Make them any size you like, too. Dip your finger in a bit of water and tap the tops of the limes to make them appear smoother.

Pipe on a small “V” shape at the top of each of the limes. This will make it appear like they have a stem.