Whisk together the egg whites and sugar in a stainless steel bowl. Place bowl in a water bath set over low heat. Whisk occasionally until sugar is dissolved and mixture is very warm, about 150 degrees F. Remove from water bath and whip with a hand-held mixer on high speed until stiff peaks form. Lower speed and add the butter in small pieces. Add vanilla and violet essence and mix until smooth. Add violet color, 1 drop at a time, until desired shade is reached. Use to fill cupcakes.