Mix cornstarch and powdered sugar in small bowl. Coat 9" x 13" baking dish with cooking spray and dust inside of baking dish with ½ the cornstarch mixture (reserve remaining mixture).
Pour ½ cup water into microwave safe bowl and add gelatin. Let stand 10 minutes.
Mix ½ cup water, sugar, agave syrup and salt in medium saucepan. Bring to boil, stirring until sugar dissolves. Attach candy thermometer to saucepan and continue to cook until temperature reaches 235 degrees.
Put bowl with gelatin into microwave and heat 30 second then pour gelatin mixture into mixing bowl of stand mixer. Mix on low speed using whisk attachment. When sugar mixture reaches 240 degrees, add to mixing bowl with gelatin mixture. Begin mixing on low, turn to high speed once everything is combined. Beat 8 minutes until mixture is thick and fluffy. Stir in vanilla and beat an additional 2 minutes on high speed.
Spread marshmallow mixture into pan using spatula (use a wet spatula to prevent mixture from sticking). Sift remaining powdered sugar mixture over top of marshmallow fluff. Let sit 5 hours at room temperature or overnight, covered.
Unmold marshmallows on cutting board, lightly coated with powdered sugar. Cut into small cubes (spray the knife with cooking spray before slicing to prevent marshmallows from sticking).
Melt chocolate per package instructions. Drizzle marshmallow cube in melted chocolate, caramel sauce (is using) then in crushed pecans. Set on plate, coating side up. Repeat steps until all cubes are decorated. Cool in refrigerator until chocolate sets.
Try cutting your marshmallow squares with a pizza cutter, and don't worry about making them perfect!