In a mixing bowl, combine butter, confectioners’ sugar, and milk. Beat at low speed with a mixer, and gradually increase to high speed, until smooth and creamy. Add melted chocolate, beating until incorporated.
Use immediately, or transfer to an airtight container and refrigerate for up to a day. Let come to room temperature before using.
Recipe by TeaTime Magazine at https://teatimemagazine.com/orange-macarons-with-dark-chocolate-buttercream/