In a stand mixer fitted with the whisk attachment (or a mixing bowl with a handheld electric mixer) beat the butter on high until it is light and fluffy. Gradually add in the powdered sugar until incorporated. Then, with the mixer on high, whisk in the milk and the Fiori di Sicilia extract until smooth. Then add in the heavy cream and whip until buttercream is light and fluffy. Use an offset spatula to smear the buttercream on the cupcakes or load it into a pastry bag with a big tip to pipe on the frosting.