Place chocolate in top of double boiler; bring water to a boil.
Reduce heat to low and cook until chocolate melts.
Combine cream cheese and sugar, beat until smooth.
Add chocolate and vanilla, beat well.
Stir in chopped pecans.
Press mixture into a lightly greased 8 inch square pan.
Chill until firm and cut into squares.
Top each square with a pecan half.
Store in refrigerator.