This cake is a riff on Key Lime Pie, complete with a tart key lime curd icing and buttery graham cracker crumbs.
- For the Cake:
- Baking spray
- 2 1/4 cups (about 11 1/4 ounces) all-purpose flour
- 2 teaspoons baking powder
- 1 teaspoon baking soda
- 3/4 teaspoon salt
- 1 cup buttermilk, at room temperature
- 2 teaspoons pure vanilla extract
- 2 sticks (8 ounces) unsalted butter, at room temperature
- 3 large eggs, at room temperature
- 1 3/4 cups (about 12 1/4 ounces) granulated sugar
- For the Graham Cracker Crunch:
- 1 1/4 cups graham cracker crumbs
- 2 ounces (1/2 stick) unsalted butter, melted and slightly cooled
- 1/4 cup (about 1 3/4 ounces) packed light brown sugar
- 1/4 teaspoon salt
- For the Key Lime Icing:
- 1/2 cup (about 3 1/2 ounces) granulated sugar
- 6 large egg yolks, at room temperature
- 1/8 teaspoon salt
- 2 tablespoons finely grated zest and 3/4 cup juice from 12 Key limes or 8 limes
- 1 (14-ounce) can sweetened condensed milk
- 1 (.25-ounce) package unflavored gelatin
- 3 tablespoons water
- 2 ounces cream cheese, at room temperature
- 2 tablespoons confectioners’ sugar
- 1/2 cup heavy cream
For the Cake: Adjust oven rack to middle positions and preheat oven to 350°F. Coat 13- by 9-inch baking pan with baking spray. Whisk flour, baking powder, baking soda, and salt together in medium bowl; reserve. Combine buttermilk and vanilla in glass measuring cup; reserve.
Beat butter and sugar on medium speed until light and fluffy, about 2 minutes. Add eggs, one at a time, beating well after each addition. Decrease speed to low; add flour mixture in three additions, alternating with buttermilk mixture, scraping down sides and bottom of bowl as necessary. Scrape batter into prepared pan.
Bake until tester inserted in center of cakes comes out clean, about 30 minutes. Transfer cake to cooling rack and cool completely in pan, 1 hour.
For the Graham Cracker Crunch: Adjust oven rack to middle position and preheat oven to 350°F. Line of 8- by 8-inch cake pan foil, allowing some excess to hang over sides. Combine cracker crumbs, melted butter, brown sugar, and salt in medium bowl, stirring until crumbs are thoroughly saturated with butter. Press onto bottom of prepared cake pan.
Bake until golden, 10 to 12 minutes. Transfer to cooling rack and cool completely, 15 to 20 minutes. Break into irregular pieces.
For the Key Lime Icing: Whisk sugar, egg yolks, and salt together in medium heavy-bottomed saucepan until thick and pale. Add lime zest and juice and cook over medium heat, whisking constantly, until thickened and the consistency of pudding, 8 to 10 minutes.
Remove from heat and whisk in sweetened condensed milk. Place a piece of plastic wrap directly on the surface of the curd and cool completely, about 1 hour. Alternatively, place the curd in an ice bath and whisk until cooled, about 10 minutes. Curd may be prepared up to 2 days in advance and refrigerated in an airtight container.
Combine gelatin and water in small bowl and allow to soften, about 5 minutes. Microwave 45 seconds and allow to cool to room temperature, 5 to 7 minutes.
Beat cream cheese and confectioners’ sugar on medium speed until light and fluffy, about 2 minutes. Add gelatin and beat to combine, about 30 seconds. Add cream and beat to soft peaks, about 2 minutes. Add curd and beat to combine, about 30 seconds.
To Assemble: Spread Key lime topping evenly over cake. Sprinkle with graham cracker crunch. Refrigerate for 1 hour to allow to set. Serve.
13- by 9-inch cake pan, electric mixer, rubber spatula, cooling rack, medium heavy-bottomed saucepan, whisk, 8- by 8-inch baking pan