Most of the bakery cupcakes that I tried that were allergen-free had frosting that was really hard and almost the consistency of a crayon when cold. It would pop off the cupcake. This has the texture of regular buttercream even if refrigerated. It does not get hard in the fridge.
The secret ingredient is the Organic Whipped Earth Balance margarine. Yes, they make non-organic too, but unless otherwise certified, corn, canola and soy oil know that are not organic are, as of this writing, in the United States, typically GMO.

The problem with most margarine or palm oil based frostings is that they can’t be aerated (I tried and it’s a fascinating if somewhat pricey science experiment in watching the failure of an emulsion). Because this margarine is already aerated it behaves like butter that has had air whipped in resulting in frosting with the “right” consistency straight from the refrigerator.

1 cup unsweetened cocoa powder
3c organic powdered sugar (or 100% Cane sugar). Sugar not labeled cane is typically from beets which are frequently GMO.
¾ c whipped Organic Earth Balance Margarine
1/3 c non-GMO creamer (coconut or soy…I use So Delicious or Silk) plus up to 3 tb more)
¼ tsp coffee granules
½ tsp salt
1 tsp vanilla

Electric Mixer
Flexible Spatula
Measuring cups
Measuring spoons

Remove Organic Earth Balance Margarine from the refrigerator and let it warm up on the counter until it is easy to pack into a measuring cup.
Sift powdered sugar if lumpy.
Measure out margarine
Put margarine in mixer and add 1 cup of the powdered sugar
Combine on low speed
Slowly add remaining sugar and cocoa powder. Mixture will be too stiff
Dissolve salt and coffee granules in coconut or soy cream and slowly pour into mixer while running.
Mix at medium until desired consistency is reached. Do not overmix.

My experience is that this stuff keeps for months in the refrigerator, which is perfect for birthday parties. Yank a cupcake out of the freezer, frost it and serve.