Preheat oven to 350 degrees. Line cupcake pan with paper baking cups. Melt butter or margarine; cool to room temperature. In a large mixing bowl cream butter and sugars. Add beaten eggs one at a time; beat well after each. Add vanilla. In a small bowl stir salt into flour; gradually add to creamed mixture. Blend thoroughly. Stir in the chocolate chips. Fill baking cups. Bake for 20–25 minutes. Cool completely before frosting. For frosting, in a mixing bowl cream together softened butter or margarine and 1-1/2 cups powdered sugar. Beat in the remaining 1-1/2 cups powdered sugar, salt, vanilla, and milk. Beat until smooth.