Makes one 9-inch layer cake
This cake has several components, most of which can (and should) be made ahead. Final assembly will ideally take place the same day the cake is served, but the cake can be stored in the fridge for up to 2 days.
Key lime curd (below)
Prepare key lime curd according to directions below at least one day (up to 3 days) ahead.
Bake cake layers according to directions below. Cool completely before frosting.
Prepare the Lime Swiss Meringue Buttercream according to directions below.
To assemble, place one cake layer (top side up) on a cake plate or board. Place about 3/4 cup of buttercream in a piping bag with a large round tip. Pipe frosting around top edges of cake to create a border of frosting. Fill in center with 1/2 cup or more of Key Lime Curd. Place other cake layer (top side down) on top.
Spread of thin layer of buttercream over top and sides of cake to form a crumb coat. Refrigerate cake for 30 minutes to 1 hour.
Place 1/4 to 1/2 cup of frosting in a piping bag with a star tip. Spread the remaining frosting evenly over top and sides of the cake. Press Graham Cracker Crumble into sides of cake. Spread about 1/2 cup of Key Lime Curd over top of cake, leaving 1/2-inch border. Pipe stars around top edge border.
Adapted from Gourmet
I made a large batch of curd not knowing how much I would end up needing for final cake assembly. As pictured and described above, I only needed half of the curd for the cake. However, if you plan to make your cake 4 layers you will probably need the whole batch.
This recipe is very easy, but the key is patience. You must cook the curd on LOW heat very slowly and you must whisk constantly. For reference, my electric stove burners range from 1 to 9. I cooked my curd on 3-4 for 10+ minutes.
1 1/2 cups granulated sugar
Add all ingredients except butter to a saucepan over Low heat. Whisk to combine. Add butter. Continue whisking gently but constantly, heating slowly, until curd thickens and reaches 160°F on an instant read thermometer. Remove from heat.
For the smoothest curd, pour through a fine mesh strainer. Transfer to a storage container. Cover and refrigerate overnight before use. Curd keeps up to 1 week in the fridge.
Adapted from Baking: From My Home to Yours by Dorie Greenspan
2 1/4 cups cake flour
Preheat oven to 350°F. Spray two 9-inch round cake pans with baking spray.
Whisk together cake flour, baking powder and salt in a small bowl. In a 2-cup measuring cup, first measure out buttermilk, the whisk in egg whites and lime juice.
In the bowl of a stand mixer, beat sugar and zest on low until fragrant, about 2 minutes. Add butter and beat until light and fluffy, about 3 minutes. Beat in vanilla extract. With the mixer on low, add 1/3 of the flour mixture, followed by 1/2 buttermilk mixture. Continue alternating dry and wet ingredients (ending with dry ingredients), beating just to incorporate each new addition. Once all ingredients are added, beat on medium speed for 2 minutes to aerate the batter.
Divide batter evenly between pans. Bake at 350°F for 30-35 minutes, or until toothpick in the center comes out clean. Cool in pans for 5 minutes, then invert cakes onto a cooling rack to cool completely.
Can be made ahead. If making ahead, wrap each layer tightly in plastic wrap and place in a zip top bag. Can be stored at room temperature for a day, or frozen for several weeks.
6 graham crackers
Preheat over to 350°F. Line a baking sheet with a Silpat or parchment paper.
Break graham crackers into quarters and pulse in a food processor until reduced to fine crumbs. Add salt and brown sugar. With processor running, pour in melted butter. Pulse until mixture resembles wet sand.
Dump mixture onto prepared pan and press mixture firmly to form a flat ‘crust’. Bake at 350°F for 6 minutes. Remove from oven and crumble into pieces with a fork. Return to oven and bake 2 more minutes. Cool completely. Can be made ahead and stored in an air-tight zip top bag until needed.
Makes about 5 cups, or enough to frost a 9-inch layer cake
If you end up making this a 4-layer cake, you may want to make an extra bit of frosting. As is, it was enough for me to frost a 2-layer cake.
6 large fresh egg whites
Fill medium saucepan with about an inch of water and bring to a simmer. Add egg whites and sugar to the metal bowl of a stand mixer. Set bowl over simmering water and whisk constantly until sugar has dissolved and mixture reads 140°F on an instant read thermometer.
Attach bowl to stand mixer and using the whisk attachment, beat on medium speed until a glossy white meringue with stiff peaks forms and the bowl no longer feels warm to the touch (about 10-15 minutes). Switch to the paddle attachment. Beating on medium-low speed, add butter one piece at a time, letting each piece incorporate before adding the next. Once all butter is incorporated, scrape down sides of the bowl with a spatula. Continue beating for 10-15 minutes until smooth.
Once smooth, add vanilla, lime juice, salt and zest. Beat until smooth.
(If at any point your buttercream looks curdled or soupy – don’t panic and for pete’s sake don’t throw it out! If it looks curdled, just keep beating. It will come together, I promise. If you’ve been beating it for 15 minutes and it’s still soupy, pop the whole bowl in the fridge for 10 minutes, then continue beating. Repeat as much as necessary until you get a smooth buttercream.)