In the bowl of a food processor fitted with blade attachment, add almond paste and 1/2 cup confectioner's sugar; pulse until almond paste and sugar are well mixed and texture resembles finely ground cornmeal. In a large bowl, cream butter and remaining confectioners' sugar until light and fluffy. Add almond paste mixture and beat until fully incorporated. Beat in the egg, milk and vanilla. Combine flour and salt; gradually add to creamed mixture and mix well. Cover and refrigerate for 1 hour.