Triple Lemon Layer Cake

Make lemon curd:

1. In a bowl, rub lemon zest into granulated sugar to release the flavorful oils. Whisk in cornstarch and salt until evenly incorporated.
2. Transfer to a medium, heavy-bottomed saucepan set over low heat. Whisk in eggs until smooth, then whisk in lemon juice.
3. Continue to whisk regularly over low heat until the mixture starts to thicken. You can switch to a spatula at any point if you like. Continue to cook until curd reaches 170ºF and thickens enough that you can swipe your spatula through it and it will not fill in for at least 3-5 seconds.
4. Remove from heat and whisk in butter, 1 cube at a time, whisking until fully melted and incorporated before adding the next.
5. Strain through a fine mesh sieve to remove the zest and any bits of cooked egg (this will give you a beautifully smooth texture). You should have about 1 cup of curd (half will be used to flavor the buttercream, the other half filling between the layers).
6. Let curd cool completely, or, if not using right away, press a layer of plastic wrap onto the surface of the curd and refrigerate overnight or for up to 5 days.

Make cake layers:

1. Preheat oven to 350ºF. Butter and line three 6-inch round baking pans with parchment paper; lightly butter parchment.
2. In a bowl, rub lemon zest into granulated sugar to release the flavorful oils. Add to large mixing bowl or stand mixer bowl along with flour, baking powder, and salt; mix briefly to combine.
3. Sprinkle butter pieces over top of flour mixture, then mix with the paddle attachment until butter is broken up into pea-sized or smaller pieces and mixture seems sandy.
4. With mixer running on low, drizzle in lemon juice, followed by 1/4 cup of milk. Increase mixer to medium-high and beat for a solid 3 to 4 minutes until liquid is fully incorporated and batter is very light and fluffy. Scrape down the sides of the bowl.
5. Meanwhile, whisk eggs and vanilla into remaining 1/4 cup milk. With mixer running on low, add egg mixture in 2 to 3 additions, mixing well after each one, until batter is evenly mixed, scraping down the sides of the bowl as needed. The batter may appear slightly curdled, that's ok.
6. Divide evenly among prepared pans (approximately 250 grams of batter in each of the three 6-inch pans if you want to be precise).
7. Bake for 22 to 25 minutes (slightly longer if making a larger size cake or if making two layers instead of three), rotating the pans half way through, until a toothpick inserted in the center comes out clean. Remove from oven and place on a wire rack until cool enough to handle. Run a small knife around the edges of each pan, then gently invert onto wire racks. Cakes should come out cleanly. Allow to cool completely. Wrap cooled cake layers in plastic wrap and refrigerate at least an hour or overnight (since cold cakes are easier to work with).
8. Alternatively, the cake layers can be frozen until ready to use, up to 1 month. Simply wrap each layer individually in a double layer of plastic wrap and then store inside a large zip top bag.

Make Swiss meringue buttercream:

1. Combine egg whites, sugar and salt in a double boiler (or use a large, heat-proof mixing bowl set over a pot of gently simmering water–the bottom of the bowl should sit over, but not touch, the water below). Whisk vigorously until doubled in volume and sugar is completely dissolved (will read 160 degrees F on an instant-read thermometer).
2. Return bowl to stand mixer fitted with the whisk attachment. Beat on medium-high speed until meringue is about tripled in volume and forms stiff, glossy white peaks. At this point it should also feel cool to the touch (measuring 80ºF or below) - this is the most important thing to check, as your buttercream will curdle if the meringue is still too warm.
3. With the mixer running on medium speed, add 1 chunk of butter at a time, increasing speed and mixing until butter is completely incorporated before adding the next piece. Your buttercream will start to soften, going from stiff peaks to soft and almost soupy, but as you continue to add butter it will thicken up again. If at any point it begins to curdle, stop adding butter and increase mixer speed to high, beating until it comes together again before adding the remaining butter.
4. If frosting is still soupy after adding all the butter (very likely if your kitchen is on the warm side) pop it in the fridge for 15 to 20 minutes to firm it up a bit, then return to the mixer and beat on medium-high for a few minutes until thick, smooth, and fluffy.
5. On the other hand, if your frosting seems too thick and almost greasy, it's likely too cold. In which case, gently melt about 1/4 cup of the buttercream in the microwave (10-15 seconds on half power), then drizzle into mixer running on low. Increase speed to medium-high and beat for 2 to 3 minutes or until light and fluffy.
6. Switch over to the paddle attachment and add 1/2 cup of the lemon curd. Mix on low speed for a few more minutes until curd is fully incorporated (mixing on low speed will also help remove excess air bubbles).
7. Buttercream can be made ahead of time, refrigerate with a layer of plastic wrap pressed to the surface. Before frosting, let come to cool room temperature then return to the stand mixer and re-whip on medium-high speed for 3 to 5 minutes or until light and fluffy once again.

To Assemble:

1. To assemble, place one layer on a cake stand or serving platter. Pipe a ring of buttercream around the edge of the cake layer, then spoon or pipe about 1/4 cup of lemon curd in the middle of the ring. Repeat with the second layer, then top with final cake layer, pressing lightly to adhere.
2. Cover the entire cake with a thin layer of buttercream. This crumb coat will make frosting the cake easier. Refrigerate for about 15 minutes to allow this crumb coat to set.
3. Remove cake from refrigerator and frost with remaining buttercream, reserving some for piping decorative borders or details if desired.
4. Slice and serve immediately, or refrigerate until ready to serve (let cake sit at cool room temperature for up to 30 minutes before serving). Refrigerate leftovers in an airtight container (or press a piece of parchment/plastic to the cut sides to keep them sealed for up to 3 days.