Spread sesame seeds on baking sheet; bake in 350F oven for 15 minutes or until golden.

Reserve half of the seeds; sprinkle remaining seeds over well-greased 13x9 cake pan; set aside.

In saucepan, bring butter, sugars and water to a boil, stirring constantly.

Cook, stirring often, until candy thermometer reaches soft-crack stage of 285F and when 1/2 tsp syrup dropped into very cold water separates into threads that are hard but not brittle.

Immediately remove from heat; stir in baking soda.

Pour over sesame seeds in prepared pan; let cool 5 minutes.

Sprinkle with reserved sesame seeds, pressing lightly into toffee.

Let cool until firm; break into pieces.

Store toffee between sheets of waxed paper in an airtight container; keeps for about 1 month.