In a saucepan mix sugar, butter, water and corn syrup. Bring to a boil over medium to medium-high heat, stirring often. Using a candy thermometer, keep cooking until liquid gets to the hard crack stage (300 degrees). The mixture should be a light caramel color. Remove from heat and stir in vanilla.
Pour mixture onto a baking sheet lined with aluminum foil. Sprinkle chocolate chips over hot toffee; spread with a spatula. Before chocolate hardens, sprinkle with pecans. Cool completely. Break into pieces. Store in airtight container with waxed paper between the layers.