Preheat the oven to 325 degrees F. Grease two 6-inch cake pans.
In the bowl of an electric stand mixer fitted with a paddle attachment, mix the cake flour, sugar, salt and baking powder. Mix until combined. In a separate bowl, mix 1/4 cup plus 2 3/4 tablespoons water, the egg whites, sour cream, lime zest and lime juice. In another small bowl, mix together the butter and oil until it reaches a mayonnaise-like consistency. Add half of both the sour cream and butter mixtures to the flour. Mix on medium speed for 1 minute. Scrape down the sides of the bowl and add the rest of the sour cream and butter mixtures and mix on medium speed until fully mixed together, 2 minutes.
Divide the cake batter between the prepared cake pans, filling each to about two-thirds full. Bake until cooked through and a toothpick inserted into the center comes out clean, 20 to 24 minutes.
Let the cake set in the refrigerator for 1 hour or in the freezer for 30 minutes before applying icing.
Trim the tops of each cake and cut each widthwise into 2 layers. This will give you 4 layers of cake. Starting with the bottom layer, brush with the Tequila Simple Syrup. Using a pastry bag filled with Salted Lime Buttercream, pipe a thin ring around the outside edge of the cake, this will keep your curd from oozing out. Spread a thin layer of Lime Curd inside ring. Use the pastry bag to pipe a full layer of buttercream on top of curd, about 1/4-inch thick. Place the next cake layer on top and repeating the process with the syrup, buttercream and lime curd and repeat the process with the third layer of cake, ending with the final cake layer on top of the buttercream. Remove the excess filling from the outside of cake with an offset spatula. Refrigerate for 15 minutes to set. Once the cake is cool, ice with the remaining Salted Lime Buttercream and serve.
Combine 1/2 cup water with the sugar in a pot, heat and stir until the sugar dissolves. Add the tequila and stir until combined. Set aside to cool.
In a small pot combine the sugar and 1/2 cup water and bring to a boil. Continue cooking until the mixture reaches the soft-ball stage. Use a candy thermometer to monitor temperature. Meanwhile, in a separate bowl, use an electric mixer to whip the egg whites and cream of tartar on medium speed to a soft peak. When the sugar syrup reaches 225 degrees F, add it slowly to the whipped egg whites, continuing to beat on medium speed until cool. While the mixer is running, add the butter a few pieces at a time. Add the lime juice, salt and vanilla extract. Mix until combined.
In a large metal bowl, combine the sugar, lime juice and lime zest. Whisk the eggs into the lime mixture and set the bowl over a pot of boiling water. Stir constantly until the curd starts to thicken, about 10 minutes. Strain the curd through a mesh strainer into another bowl. Add the butter a few cubes at a time and whisk until smooth. Set aside to cool, stirring frequently.
Once the curd is cool, keep it in the refrigerator in an airtight container.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.