Cake Mix Hack: Gluten-Free Key Lime Pie Cupcakes

Now that I am officially a PokemonGo widow, I can finally get around to sharing this Cake Mix Hack!  I made these cupcakes ages ago for my coworker’s birthday.  He casually dropped the idea of key lime pie for his birthday and then I set my mind on figuring out these cupcakes.  With a light lime flavor in the cake itself, a smooth lime curd filling, and a tangy frosting, these cupcakes are (a) super delicious and (b) full of that delectably tart flavor akin to Key Lime Pie.  

I think these cupcake would be the perfect end to your next summer picnic!

Key Lime Pie Cupcakes

Serves: 12 

Cakes:

• 1 box Gluten-free yellow or white cake mix + additional ingredients listed on the back of the package*
• 2 Tbsp. key lime juice, divided
• zest of 1 lime
• 2 Tbsp. sugar
• 1 Tbsp. hot water

Lime Curd:

• 1/2 c. lime juice**
• 1/2 c. sugar
• 2 eggs
• 1/2 stick (4 Tbsp) butter, melted and cooled.
• zest of 2 limes
• pinch of salt

Frosting:

• 1/2 package(4 oz) cream cheese, room temperature
• 1/2 stick (4 tbsp) butter, room temperature
• 2-3 c. confectioner’s (powdered) sugar
• 2 tsp. key lime juice
• zest of 1 lime
• optional: 1 tsp. vanilla extract***

*I used Aldi’s GFree yellow cake brand, which calls for 3 eggs, 2/3 c. milk, and 1/2 c. oil per box

**I juiced all the normal (non-key) limes that I had zested for the various parts of this recipe, and then topped off the remainder with key lime juice, so I suppose the lime curd isn’t officially a “Key Lime” Curd.  Waste not, want not!

***These individual elements add up to a deliciously tart cupcake.  If you are worried it might be too tart for your tastes, adding the vanilla extract will help to mellow the frosting.

Make and bake the cakes:

Preheat the oven according to package directions (the Aldi box called for 350 degrees F).  Line a cupcake tin with paper liners and lightly spray the liners with cooking spray.

Measure out the eggs and wet ingredients called for on the package, except subtract 2 tablespoons of the liquid (not fat!) addition.  My package called for 2/3 c. milk, so I measured off 2 tablespoons of milk after pouring 2/3 cup of milk.  We are doing this because we are adding in lime juice to the cake mix and we want to keep the ratio for the batter correct.  Gently mix the liquid (milk), fat (oil), and eggs called for on the packaging, and then add to the dry ingredients.  Stir until just mixed.  Add in lime juice and zest, and stir until just mixed.  Scoop batter into prepared cupcake tins and bake according to package directions (my box said 18-23 minutes).

While the cupcakes are baking, mix the sugar with the hot water and remaining tablespoon of lime juice to make a lime simple syrup.  Stir until sugar is dissolved.  When cupcakes are done baking, remove cake tins from the oven.  Using a toothpick, poke half a dozen holes into the top of each hot cupcake, then brush with simple syrup.  Allow to cool in cupcake tins for 10 minutes, then carefully remove and cool on a baking rack until cupcakes reach room temperature.

Make the lime curd (in the microwave!):

While the cupcakes are baking, make the lime curd.  Mix the eggs and the sugar until thoroughly combined.  Whisk in lime juice, butter, lime zest, and salt.  Microwave on high for 1 minute.  Remove from microwave and stir well.  Continue to microwave for 1 minute intervals, whisking well in between each minute, until the curd thickens and coats the back of a spoon.  This can take anywhere between 6-10 minutes.  The curd will thicken slightly as it cools.  Cool lime curd completely before filling cupcakes.

Make the frosting:

While the cupcakes and lime curd cool, make the frosting.  In a mixer, mix together softened cream cheese and butter on medium-low speed until thoroughly combined.  Add the lime juice, zest, and vanilla extract (if using) and mix on low until combined.  Add the confectioner’s sugar in 1/2 cups, mixing well between each addition.  Finish off the frosting by whipping on high speed for one minute to make it fluffy!

Assemble the cupcakes:

Using a small cookie cutter (or moving very carefully with a butter knife), cut a small hole in the middle of each cupcake.  Make sure to not cut through to the bottom of the cake.  I used the cutter for an inner window of a linzer cookie cutter set, which was the perfect size.  Gently remove the cut portion.  You should end up with a little cap, and a hole in your cupcake.  Gently push down on the bottom of the hole in your cupcake (I used the end of a chopstick) to make a little more space to be filled with lime curd.  Spoon about 1 1/2 teaspoons of curd into the cupcake, then top with the cake cap.  Don’t worry if the cap protrudes slightly–it will be hidden by the frosting.  When all the cupcakes are filled with curd and capped, frost in your preferred manner.  I’ve yet to master piping, so I just used a knife.  Optionally, sprinkle the frosted cupcakes with green sprinkles or more lime zest as decoration!

These cupcakes are definitely time-intensive, though they are decadent enough to make up for it.  I baked and assembled them over the course of two days.  Please remember that the lime curd and frosting should be refrigerated for any prolonged period (more than an hour or two) and that GF cake gets hard and dense when refrigerated.  If at all possible, wait to fill and frost these cupcakes as close to serving as possible, so they needn’t be refrigerated.  If you must refrigerate the entire cupcake (as I had to), allow the cupcakes to come to room temperature again before serving.  Trust me, they will be so much better for it!