Prepare the Cake Pans: Preheat the oven to 350°F (175°C). Line three 8-inch cake pans with parchment paper circles, then coat them with baking spray or brush them with cake release.
Make the Graham Cracker Crust: Combine the graham cracker crumbs, melted salted butter, and granulated sugar in a bowl. Stir until the crumbs are evenly moistened. Divide the mixture equally among the prepared cake pans and press it firmly into an even layer across the bottom of each pan using a flat-bottomed cup. Bake the Crusts: Bake the graham cracker layers for 8 minutes to help them set. Remove the pans from the oven and set them aside while preparing the cake batter.
Combine the Dry Cake Ingredients: In a large mixing bowl, whisk together the cake flour, granulated sugar, baking powder, baking soda, and salt until evenly distributed.
Incorporate the Butter: Using an electric mixer on low speed, add the room-temperature cubed butter one tablespoon at a time. Mix each addition into the flour mixture before adding the next. Continue until all the butter has been incorporated and the mixture resembles dry, sandy crumbs. Add the Oil: Pour in the vegetable or canola oil. Continue mixing on low speed until the flour mixture is evenly moistened.
Finish the Cake Batter: Add the eggs, room-temperature buttermilk, lime zest, key lime juice, and vanilla extract. Mix until the ingredients are evenly combined, stopping as needed to scrape the bottom and sides of the bowl with a rubber spatula.
Fill the Cake Pans: Divide the batter evenly among the three prepared pans, spreading it over the baked graham cracker crusts.
Bake the Cake Layers: Bake for 28–32 minutes, or until a toothpick or skewer inserted into the center of each cake comes out clean or with only a few crumbs clinging to it. Allow the layers to cool completely before assembling the cake. To prepare them ahead, wrap the cooled layers tightly in plastic wrap and freeze for up to 2 months. Combine the Key Lime Curd Ingredients: In a medium saucepan, whisk together the granulated sugar, key lime juice, lime zest, and egg yolks until combined. Add the salted butter.
Cook the Key Lime Curd: Place the saucepan over medium-low heat. Cook, whisking or stirring constantly, for about 10–14 minutes, until the butter melts and the curd thickens enough to coat the back of a spoon.
Strain and Cool the Curd: Pour the curd through a fine-mesh strainer to remove any lumps. Cover it with plastic wrap pressed directly against the surface to prevent a skin from forming. Let it cool completely or refrigerate it until ready to assemble the cake. Beat the Frosting Base: Place the salted butter and full-fat cream cheese in a large mixing bowl. Beat with an electric mixer until completely combined and smooth.
Finish the Frosting: Add the powdered sugar, key lime juice, vanilla extract, and lime zest. Mix just until the frosting is smooth, fluffy, and spreadable. Avoid overmixing, as the frosting can become runny.
Level the Cake Layers: Once the cake layers are completely cool, trim their tops if necessary to create even surfaces.
Prepare the First Layer: Place one cake layer on a cake plate with the graham cracker crust at the bottom. Spread a thin layer of frosting over the cake surface to help prevent the key lime curd from soaking into the cake.
Pipe a Frosting Border: Transfer 2 cups of frosting to a piping bag fitted with a large open tip. Pipe a continuous border around the top edge of the first layer to create a barrier that will hold the curd in place.
Add the First Layer of Curd: Spread half of the cooled key lime curd evenly inside the frosting border.
Stack the Second Layer: Place the second cake layer on top. Spread a thin layer of frosting over it, pipe another border around the edge, and distribute the remaining key lime curd evenly inside the border.
Add the Final Cake Layer: Position the third cake layer on top. Cover the top and sides of the assembled cake with the remaining frosting, reserving enough for decorative swirls around the top edge.
Decorate the Cake: Press extra graham cracker crumbs into the frosting around the lower third of the cake. Using a piping bag fitted with a Wilton 1M tip or another decorative tip, pipe frosting swirls around the top edge. Finish with fresh lime slices.
Chill Before Serving: Freeze the assembled cake for 20 minutes or refrigerate it for 1 hour to help the frosting set. Slice and serve once the cake is ready.
