For easiest assembly, place the buttercream in a pastry piping bag fitted with a tip.
This marshmallow filling should be used immediately, so it its best to wait to make it until the cakes are almost cooled.
Place the egg whites, sugar and corn syrup in the top of a double boiler over rapidly boiling water. Beat constantly with an electric mixer until soft peaks form. It should take 3-4 minutes.
Transfer the mixture to a large mixing bowl or the bowl of a stand mixer.
Beat on medium high speed, with a whisk attachment if you have it, until soft peaks form. It should take less than a minute.
Transfer to a pastry piping bag fitted with a tip for easiest assembly.
Assemble the Whoopie Pies
Take one of the little cakes and add frosting to the flat bottom. Gently press another little cake on top. ENJOY!
*I tested this recipe with King Arthur Measure for Measure Flour which contains xanthan gum. If you use a gluten-free flour blend without binders like xanthan gum, guar gum or inulin, add 1/2 tsp xanthan gum.
Did you make this recipe?Let me know how it was! Tag @chefjanetk on Instagram.