Preheat your oven to 350°F (175°C). Prepare three 6-inch cake pans by greasing them lightly and lining each with parchment paper.

In the bowl of a stand mixer, combine the granulated sugar and lime zest. Rub together with your fingertips until the sugar becomes fragrant and resembles damp sand.

Add the cake flour, baking powder, and salt into the sugar mixture, blending on low speed.

Cut the butter into cubes and add it gradually to the dry ingredients, mixing until the texture is like fine sand.

Pour in the oil, milk, and vanilla extract, and mix just until everything is combined.

In a separate bowl, use a hand mixer to whip the egg whites until they reach medium peaks.

Gently fold the whipped egg whites into the batter using a rubber spatula, mixing carefully until just incorporated.

Evenly distribute the batter among the prepared cake pans.

Bake for approximately 22-25 minutes, or until the edges of the cakes pull slightly away from the pans. Transfer the cakes to a wire rack to cool completely.

Once cool, wrap each layer in plastic wrap and freeze until you’re ready to frost and decorate. (Frozen cake layers make for easier frosting.)

In a medium bowl, whisk together the egg yolks, sugar, cornstarch, and salt until the mixture is smooth and slightly lightened in color. Stir in the key lime juice until thoroughly blended.

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

Add the egg yolk and lime juice mixture to the melted butter. Raise the heat to medium, stirring constantly with a rubber spatula for 3-4 minutes, until the curd thickens and bubbles form at the center.

Remove from heat immediately, pouring the curd through a fine-mesh sieve into a clean bowl. Stir in the key lime zest, transfer the curd to a jar, and chill for at least 1 hour or preferably overnight.

Begin by wiping down the bowl of a stand mixer and the whisk attachment with a bit of white vinegar to remove any grease.

Place the egg whites, sugar, and salt in the bowl. Whisk together until well combined.

Set the bowl over a saucepan of simmering water, stirring frequently until the mixture reaches 160°F (71°C) or until the sugar has dissolved and the mixture is no longer grainy to the touch.

Transfer the bowl to the stand mixer with the whisk attachment, beating on medium-high until the meringue holds medium peaks.

Add the vanilla extract, then continue beating until stiff peaks form.

Lower the mixer speed and wait until the meringue cools to around 85°F (29°C).

Gradually add the butter, a few cubes at a time, ensuring each piece is fully incorporated before adding more. The buttercream may look curdled at first, but continue beating, and it will smooth out.

Once the buttercream comes together, switch to the paddle attachment and mix on low speed until it becomes smooth and creamy.

Place the first cake layer on your cake stand. Pipe a rim of buttercream around the edge, creating a barrier. Fill the center with a layer of key lime curd.

Add the second layer of cake and repeat with the buttercream dam and curd filling.

Place the final cake layer on top and add a few scoops of buttercream. Smooth the top with an offset spatula, spreading the excess buttercream around the sides.

Decorate as desired, finishing with a sprinkle of key lime zest on top. Enjoy!