1️⃣ Make the Lemon Curd:

Combine sugar and lemon zest in a bowl, rubbing together with your fingers to release oils. 🥣🍋
In a heatproof bowl, whisk together egg yolks, lemon-sugar mixture, and lemon juice. 🍋🥚
Place the bowl over a saucepan of simmering water, stirring constantly until thickened (10-15 minutes). 🍲
Remove from heat and stir in butter, one piece at a time, until smooth. 🧈
Cover and refrigerate until set. ❄️
2️⃣ Prepare the Lemon Cake Layers:

Preheat your oven to 350°F (175°C). Grease and line three 8-inch cake pans. 🔥
Mix together sugar and lemon zest until fragrant. 🥄🍋
Sift together cake flour, baking powder, and salt in a large bowl. 🍰
In another bowl, beat butter until creamy, then add lemon-sugar mixture and beat until fluffy. 🧈🍋
Mix in egg whites on low speed until combined. 🥚
Alternately add flour mixture and buttermilk, starting and ending with flour. 🥛
Mix in vegetable oil, vanilla extract, and yellow gel food coloring if using. 🌈
Divide batter evenly among prepared pans. 🥄
Bake for 30-35 minutes, or until a toothpick comes out clean. 🕰️
Cool in pans for 10 minutes, then transfer to wire racks to cool completely. 🌬️
3️⃣ Make the Lemon Buttercream Frosting:

In a heatproof bowl, whisk together egg whites and sugar. 🍚🥚
Place over simmering water, whisking constantly until sugar dissolves and mixture reaches 160°F (70°C). 🍲
Remove from heat and beat on high speed until stiff peaks form and mixture cools to room temperature. ❄️
Gradually add butter, one piece at a time, beating until smooth. 🧈
Mix in lemon zest, lemon juice, vanilla extract, lemon extract, and salt. 🍋
4️⃣ Assemble the Cake:

Level the cooled cake layers if necessary. 🏞️
Place one layer on a serving plate, spread with a thin layer of lemon curd, then a layer of buttercream. 🍋🍰
Repeat with remaining layers, then frost the top and sides of the cake with buttercream. 🎂
Chill for at least 30 minutes before serving. ❄️