Melt chocolate and butter in small saucepan over low heat, stirring occasionally until melted. Stir in milk until smooth. Pour into bottom of pie crust.
Combine gelatin and cold water in a small bowl, mixing well. Let rest 5 minutes. Heat milk, sugar, salt and gelatin mixture in a small saucepan over low heat, stirring until sugar and gelatin have dissolved. Do NOT boil. Remove from heat. Refrigerate until cool and the consistency of syrup, about 30 minutes.
Beat cream and vanilla extract in a large bowl with electric mixer until stiff. Pour gelatin mixture over cream and fold in until well blended. Pour into pie crust. Refrigerate at least 1 hour until pie is firm.
Recipe note: double the chocolate mixture, reserving half to drizzle over the top of the pie (reheat if necessary).