Key Lime Pie Impossible Cake
White chocolate limes, bright and citrusy lime caramel, and a creamy key lime flan, all atop a buttery, graham cracker cake.
Don’t let the name scare you! An Impossible Cake, traditionally made with chocolate cake and caramel flan, is really quite simple to make. The “impossible” part is that the cake batter and the flan switch places in the oven! The graham cracker cake in this recipe begins on the bottom of the pan, but floats to the top as buttermilk and baking soda react to create air pockets, while the denser key lime flan sinks through it to the bottom.
Servings: 12 to 16 | Prep Time: 1 hour 15 minutes | Cook Time: 1 hour 15 minutes + 10 hours chilling
Double boiler / Heat-safe bowl
Citrus wedge mold (link)
Bench scraper / Knife
Small paintbrush
Paper towels
Bundt cake pan
Medium heavy-bottomed saucepan
Large metal whisk
Hand-mixer / Stand-mixer
Blender / Food processor
Small fine-mesh sieve
Roasting dish
Piping bag
Large star piping tip
Microplane zester
White Chocolate Lime Wedges:
3 ½ oz white chocolate chips
¼ tsp key lime flavoring oil (LorAnn, must be safe for chocolate)
Yellow food coloring
Teal food coloring
For Greasing the Pan:
2 tbsp shortening
2 tbsp flour
Key Lime Caramel:
1 cup sugar
¼ cup water
3 tbsp key lime juice (Nellie & Joe’s)
3 tbsp cold butter, cubed
Graham Cracker Cake:
1 ½ cups fine graham cracker crumbs (about 13 full sheets)
½ cup flour
1 tsp baking powder
1 tsp baking soda
½ cup + 2 tbsp butter, room temp
1 cup sugar
1 egg
1 tsp vanilla
1 cup buttermilk
Key Lime Flan:
1 (12 oz) can evaporated milk
1 (14 oz) can sweetened condensed milk
1 tsp vanilla
6 eggs
¼ cup key lime juice
1 tbsp key lime zest
¾ tsp key lime flavoring oil
Stabilized Whipped Cream:
1 cup heavy whipping cream, cold
4 tbsp powdered sugar
1 tsp clear vanilla extract
4 oz cream cheese, room temp
Assembly:
Zest of 1 key lime
White Chocolate Lime Wedges (30 minutes)
Over a double boiler (or in a heat-safe bowl in the microwave), carefully melt the white chocolate until it is smooth. Take the bowl off the heat and set on a towel.
Add the key lime flavoring oil and mix.
In a small bowl, mix three drops of yellow food coloring and one small drop of teal food coloring until a lime green color is achieved. (You can get a better look at the color by brushing a small amount onto a paper towel.)
Add one drop of the green dye to the chocolate at a time, mixing in between additions, until the color matches the inside of a lime. Be careful not to add too much at a time or the chocolate may seize.
Fill the citrus wedge molds with the green chocolate and carefully scrape off any excess with a bench scraper or back of a knife.
Allow the chocolate to fully harden, about 5 to 10 minutes, and carefully remove the chocolate lime wedges from the mold.
Use a small paintbrush to brush more green dye onto the rinds of the chocolate limes.
Use a paper towel to rub the dye into the chocolate, wiping off any excess. You may need to apply a couple coats. Make sure the dye reaches all the way to the edges.
Let sit uncovered until dye has fully dried. Store in an airtight container at room temperature until ready to use.
Key Lime Caramel (10 minutes)
Preheat the oven to 350° F.
Grease the inside of a Bundt cake pan with shortening. Dust with flour and tap out excess.
In a medium, heavy-bottomed saucepan, combine the sugar and water.
Cook over high heat until amber in color, gently swirling the pan instead of stirring.
Remove from heat and, with a large metal whisk, whisk in the lime juice and cubed butter (the mixture will bubble and steam up rapidly, so be careful!)
Pour the caramel into the bottom of the Bundt pan and set aside.
Graham Cracker Cake (15 minutes)
In a small bowl, whisk together the graham cracker crumbs, flour, baking powder, and baking soda. Set aside.
In a large bowl, using a hand-mixer or stand-mixer, cream together the butter and sugar until light and fluffy.
Add in the eggs, one at a time, beating after each addition.
Then mix in the vanilla.
Beat in one third of the flour mixture until just combined, then mix in half of the buttermilk. Continue to alternate, ending with the flour. Be careful not to overmix.
Pour the cake batter into the Bundt pan on top of the lime caramel.
Key Lime Flan (10 minutes)
In a blender, or food processor, combine all the flan ingredients and blend on high until smooth, about 30 to 60 seconds.
Gently pour the flan mixture through a small, fine-mesh sieve into the Bundt pan, so it lays on top of the cake batter. If the batters mix a little, do not worry! They will separate from each other in the oven.
Place the Bundt pan in a large roasting dish and pour hot water into the roasting dish, until it reaches halfway up the sides of the Bundt pan.
Baking (1 hour 15 minutes + 10 hours cooling)
Place into the oven and bake until a cake tester, or toothpick, inserted into the cake comes out clean, about 1 hour 15 minutes.
Transfer the Bundt pan to a wire rack to cool to room temperature, about 2 hours.
Refrigerate until the flan sets, about 8 hours or overnight.
To remove the cake, place the bottom third of the Bundt pan in warm water for 5 minutes. Do not heat for too long or the flan will start to break down.
Place your serving dish upside down onto the Bundt pan and carefully flip it over.
Slowly remove the Bundt pan, allowing the lime caramel to drizzle down over the cake.
Stabilized Whipped Cream (5 minutes)
In a large bowl, or the bowl of a stand-mixer, combine the heavy whipping cream, powdered sugar, and vanilla.
Using a hand-mixer or the whisk attachment of a stand-mixer, whip on high for a few minutes or until soft peaks form. Do not overwhip.
Carefully fold in the room temperature cream cheese until well combined.
Fill a piping bag fitted with a large star tip.
Pipe a looping ring of whipped cream on top of the cake.
Assembly (5 minutes)
Using a microplane, grate the key lime zest onto the cake.
Lay the chocolate limes on their sides an equal distance apart onto the whipped cream.
Slice and serve immediately.
Enjoy!
Be sure to check out my Best Practices page for tips on how to make this recipe yourself. Let me know in the comments if you have any questions and feel free to tag me on Instagram @made.by.shade if you make it!
-Shade