For the smoothest yet pipable buttercream, make this Swiss Meringue Buttercream.
4large egg whites136 g (or 1/2 cup liquid from can of cannellini beans)
1-1/4 to 1-1/3cupsgranulated sugardivided (272 g)
1-3/4cupsroom temperature unsalted butter, each stick cut into 32 - 40 pieces(3 sticks + 4 tablespoons - 408 g) or coconut oil or vegetable shortening (Spectrum)
1teaspoonpure/gluten-free extractvanilla, mint, almond, banana, lemon, etc.
Gluten free food coloringoptional
In a saucepan over a bain marie (heat-proof bowl, preferably glass, over [not touching a saucepan of simmering water) or a double boiler, whisk together the egg whites and sugar. Continue to whisk until all of the sugar melts and mixture reaches 160ºF.
Pour the egg-sugar mixture in the bowl of your electric mixer and whip on medium-high, mixing until the bowl is cool and the mixture turns into marshmallow fluff.
Add the pieces of butter one at a time, no more than three making sure they are thoroughly blended and smooth before adding more.
Transfer to a piping bag and pipe as desired or using an off-set spatula, frost a cake.
Refrigerate any leftovers up to 5 days or freeze up to 5 months, tightly sealed.