Combine chocolate and butter in top of a double boiler; cook over boiling water until melted, stirring occasionally. Remove from heat; cool.
Add chocolate mixture to eggs; beat well. Add sugar, flour, milk, salt, and vanilla; beat until well blended. Pour mixture into a well-greased 9-inch pie plate. Bake at 325° for 50 minutes. Serve warm with vanilla ice cream, if desired.