1. Combine sugar, corn syrup, water, salt and butter in a large saucepan. Cook, stirring constantly, until sugar dissolves. Cover pan for 1 minute to allow the steam to wash down the sugar crystals that cling to side of pan, or wipe down the crystals with a damp cloth.
2. Uncover pan; insert candy thermometer. Cook without stirring until candy thermometer reaches 270 degrees F (soft crack stage, where syrup, when dropped into very cold water, separates into hard but not brittle threads).
3. Remove form heat; stir in almonds; return to heat. Continue cooking until candy thermometer reaches 300 degrees F (hard crack stage, where syrup, when dropped into very cold water, separates into thread that are hard and brittle).
4. Remove from heat; stir in baking soda. Let foaming syrup settle just a bit, the quickly pour out onto a well-buttered large cookie sheet, and stretch out as thin as possible with the aid of two forks. Cool completely, then break in pieces. Store 2 to 3 weeks in tightly covered containers, separating the layers with foil or plastic wrap.