Stir together sugar, corn syrup, water, butter, salt, and butter flavor. Bring to simmer on medium heat and place lid on pot for one minute then remove lid. This will steam all sugar crystals on side of the pot and they will slide down into the mixture.
Place a candy thermometer in pan and continue to cook on medium heat stirring frequently until candy reaches the soft crack stage. Add nuts and mix through, continue to cook until mixture reaches hard crack stage. Remove from heat and add baking soda. Mixture will foam, make sure you blend baking soda into candy completely.
Pour on to a nonstick cookie sheet or lightly buttered baking sheet. Spread to 1/4" thick or less while the mixture is still hot, candy will start to harden quickly. When set and cool break into pieces and store in airtight container.