Hi there, Hillary! We'd recommend just adding some lemon to the cake in the Key Lime Whoopie Pies recipe! You can add some zest, try starting out with the zest from 1/2 a lemon. Another great way to add lemon flavor to that cake would be to use extract or oil, you'll just need an 1/8 to 1/4 teaspoon. We hope this helps and happy baking! 

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