Preheat the oven to 350 degrees F. Grease and line three 6 inch cake pans with parchment paper.

In the bowl of a stand mixer fitted with a paddle attachment, add the sugar and key lime zest. Rub them together with your fingers until it resembles wet sand and smells aromatic.

To the sugar, mix in the flour, baking powder, and salt.

Cut up the butter into small cubes and add to the dry ingredients. Mix on low speed until it resembles wet sand.

Add the oil, whole milk, and vanilla, and mix until just combined.

In a separate medium bowl, add the egg whites. Use an electric hand mixer to whip them until medium peaks form.

Gently fold the whipped egg whites into the batter with a rubber spatula until just combined.

Divide the cake batter between the prepared pans.

Bake for about 22-25 minutes or until the cakes start to pull away from the sides of the pans. Immediately turn the cakes out onto a wire rack to cool completely.

Once cool, wrap the cakes in plastic wrap and freeze until ready to decorate. (Frozen cake layers will be much easier to frost!)

In a medium bowl, whisk together the egg yolks, sugar, cornstarch, and salt until smooth and light in color. Then, mix in the key lime juice until fully combined.

In a medium saucepan over low-medium heat, melt the butter.

Pour in the egg yolk, lime juice mixture, then turn the heat up to medium, and mix constantly with a rubber spatula for 3-4 minutes or until thickened and large bubbles form in the center. Immediately remove from heat.

Pour the key lime curd through a fine mesh sieve into a bowl. Stir in the key lime zest, then transfer to a jar and chill for at least 1 hour or overnight.

Wipe the bowl of a stand mixer and whisk attachment down with white vinegar.

Add the egg whites, sugar, and salt to the bowl and whisk until fully combined.

Place over a small saucepan of simmering water, and whisk frequently until it reaches 160 degrees F or until the sugar is completely dissolved and the mixture no longer feels grainy.

Move the bowl over to the stand mixer with the whisk attachment. Whisk on medium-high speed until medium peaks form.

Add the vanilla and continue whisking until stiff peaks form.

Turn the mixer down to low speed, and wait until the meringue cools down to at least 85 degrees F.

Cut the butter into cubes and add to the meringue a few pieces at a time, allowing them to fully incorporate before adding more.

Once all of the butter is added, it may look curdled, but keep beating for a few more minutes and it will come together.

Once the buttercream has mostly come together, change the whisk attachment to the paddle and keep mixing on low speed until homogenous and smooth.

Place the first cake layer on a cake stand. Pipe a dam of buttercream around the edge of the cake. Fill the center with an even layer of key lime curd.

Top with the second cake layer and repeat.

Add the last cake layer and top with a few scoops of buttercream. Use an offset spatula to smooth out the top, and spread the excess all over the sides.

Continue decorating the cake as desired. Top with key lime zest, and enjoy!