Preheat oven to 350°F.
In a medium saucepan, combine lentils with 11⁄2 cups water. Bring to a boil, reduce heat, cover and simmer until lentils are tender (about 25 minutes). Drain.
Combine sugar, cocoa, flour, salt, corn syrup, eggs, melted butter, and vanilla extract in a large mixing bowl and beat for 30 seconds at medium speed; do not overbeat. Stir in cooked lentils and chopped pecans.
Pour into an unbaked pastry shell. Bake 55 to 60 minutes; immediately arrange pecan halves on top, and bake for another 5 minutes. Cool. For best flavor, cover pie and let stand a day before serving.