White Cake with Lemon-Lime Curd Filling and Whipped Cream Frosting

1. 1
   Fill a medium saucepan with 1 to 2 inches of water and bring it to a simmer over high heat; reduce the heat to medium low and keep the water at a bare simmer.
2. 2
   Place all of the ingredients except the butter in a large heatproof bowl and whisk to combine. Set the bowl over, but not touching, the simmering water and whisk constantly until the mixture thickens and is the consistency of mayonnaise, about 13 to 15 minutes. (Periodically check to make sure the water isn’t boiling by removing the bowl from the saucepan using a potholder or dry towel. If the water is boiling, reduce the heat so the eggs don’t curdle.)
3. 3
   Remove the bowl from the simmering water. Immediately whisk in the butter 1 piece at a time, waiting until each piece is completely melted and incorporated before adding the next.
4. 4
   Set a fine-mesh strainer over a large bowl. Strain the curd, pressing on the solids and scraping the extra curd clinging to the underside of the strainer into the bowl; discard the solids. Immediately press a sheet of plastic wrap directly onto the surface of the curd to prevent a skin from forming. Refrigerate until set and chilled, at least 3 hours. (The curd can be made and refrigerated up to 3 days ahead.) Meanwhile, make the candied zest, if using.

For the candied zest (optional):

1. 1
   Using a vegetable peeler and being careful to avoid the white pith, remove the zest from the lemons and limes in large strips. Very thinly slice the strips lengthwise.
2. 2
   Place the zest in a medium saucepan and add enough cold water to cover it by 1 inch. Place over medium-high heat and bring to a boil. Drain through a strainer and set the zest aside in the strainer; reserve the saucepan (no need to wash it).
3. 3
   Place 1 1/2 cups of the sugar and the measured water in the reserved saucepan and bring to a boil over medium-high heat, stirring to dissolve the sugar. Add the reserved zest (reserve the strainer—no need to wash it) and stir to combine.
4. 4
   Reduce the heat to medium low and simmer until the zest is translucent, about 30 minutes. Meanwhile, place the reserved strainer over a medium heatproof bowl.
5. 5
   Pour the mixture through the strainer (reserve the syrup for another use) and set the zest aside to cool slightly, about 3 minutes.
6. 6
   Place the remaining 1 cup of sugar in a medium bowl. Working in small batches, toss the warm zest in the sugar, coating the strips thoroughly. Shake off the excess sugar, transfer the zest to a baking sheet, and spread it into a single layer. Let cool completely, about 10 minutes, and set aside. (Once cooled and dry, the candied zest can be covered with plastic wrap and stored at room temperature for up to 5 days.)

For the cake:

1. 1
   Heat the oven to 350°F and arrange a rack in the middle. Coat 2 (8-inch) cake pans with butter and flour and tap out any excess flour; set aside.
2. 2
   Whisk the measured flour, baking powder, and salt together in a medium bowl; set aside.
3. 3
   Place 1 1/4 cups of the sugar and the measured butter in the bowl of a stand mixer fitted with a paddle attachment and beat on medium speed until the mixture is airy and light in color, about 5 minutes. Add the zest and vanilla and mix until incorporated, about 30 seconds. Stop the mixer and scrape down the paddle and the sides of the bowl.
4. 4
   Add a third of the reserved flour mixture and turn the mixer to low speed, mixing until the flour is just incorporated. Add half of the milk and mix until just incorporated. Continue with the remaining flour mixture and milk, alternating between each, until all of the ingredients are incorporated and smooth. Transfer the mixture to a large bowl.
5. 5
   Wash and dry the bowl of the stand mixer. Place the egg whites in the bowl, attach it to the stand mixer, and fit the mixer with the whisk attachment. Whisk the whites on high speed until medium peaks form, about 1 minute. Add the remaining 1/4 cup of sugar and continue whisking until stiff, glossy peaks form, about 45 seconds to 1 minute more. Stop the mixer.
6. 6
   Using a rubber spatula, fold a quarter of the egg whites into the cake batter until evenly incorporated; gently fold in the remaining egg whites until just combined and no pockets of white remain.
7. 7
   Divide the batter between the prepared cake pans and spread it into an even layer. Bake until the surface of the cakes springs back when pressed and a toothpick or cake tester inserted into the center comes out clean, about 30 minutes. Remove the cakes from the oven, place on wire racks, and let cool for 15 minutes. Run a knife around the perimeter of each and turn them out onto the racks to cool completely, at least 45 minutes.

For the frosting:

1. 1
   When ready to serve the cake, chill the bowl of a stand mixer and the whisk attachment in the freezer for 10 minutes. Add all of the frosting ingredients to the chilled bowl and whisk on high speed until medium peaks form, about 1 to 1 1/2 minutes.

To assemble:

1. 1
   Using a serrated knife, trim a very thin layer from each cake top, just enough to even out the surface. Slice each cake in half horizontally so that you have 4 layers.
2. 2
   Rewhisk the lemon-lime curd until smooth. Place a cake layer on an upside-down cake pan or serving plate. With a long knife or offset spatula, evenly spread a third of the lemon-lime curd (about 1 cup) over the top of the cake, leaving a 1/2-inch border.
3. 3
   Top with a second cake layer and evenly spread another third of the lemon-lime curd over the top of the cake, leaving a 1/2-inch border. Top with a third cake layer and evenly spread the remaining lemon-lime curd over the top of the cake, leaving a 1/2-inch border. Top with the fourth and final cake layer. (If the layers start to slide, use your hands to line them back up, then place in the refrigerator for 30 minutes to firm up.)
4. 4
   Using a clean long knife or offset spatula, coat the top and sides of the cake with the whipped cream frosting in a smooth, even layer. If using the candied zest, sprinkle half of it on top of the cake, leaving a 1-inch border. Using your hands, gently press the remaining zest into the bottom 1 inch of the cake sides. Serve immediately.