Grate the chocolate chips in a blender to a fine consistency. In a small bowl, whisk together the egg yolk with 2 tbsp. of sugar until light and lemony colored. Combine the remaining sugar and milk in a small saucepan and heat until tiny bubbles appear along the edges of the pan. Remove from heat, and pour small amount of the hot mixture into the egg; stir and pour the egg and milk mixture back into the pan. Cook over low heat until the mixture begins to thicken or reaches 185 degrees. Remove from heat and add the hot mixture to the chocolate in the blender. Add the softened butter and vanilla,, and process 1 minute or until smooth. Pour into cake pan and freeze overnight. Allow to sit at room temperature 45 minutes before serving.