This recipe, for Cinder Toffee, I have not tried but it does contain cream of tartar, which I found on a search. The recipe below is one that I have tried, without the cream of tartar, and I think it turned out fine. Hope this helps!

Cinder Toffee
Confectionary - Toffee

250 g / 1/2 lb sugar
1 T golden syrup
1/4 t cream of tartar
75 ml / 1/3 cup water
1/4 t bicarbonate of soda (baking soda)
1 t warm water

METHOD
Put the sugar, syrup, cream of tartar into a heavy pan and heat gently until the sugar has dissolved. Boil without stirring until a little of the mixture forms hard, brittle threads when tested in cold water (154C/309F on a sugar thermometer). Dissolve bicarbonate of soda in the t of warm water and pour into the toffee which will froth up. Stir well and pour into a shallow greased tin. Break into pieces when cold. Wrap in greaseproof paper and store in an airtight tin.

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Sponge Candy

1 c. sugar
1 c. dark corn syrup
1 Tbls. cider vinegar
1 Tbls. baking soda

Heat sugar, corn syrup, and vinegar to boiling in 2 qt. saucepan over medium heat, stirring constantly, until sugar is dissolved.

Boil without stirring, to 300' on candy thermometer (hard ball stage); remove from heat. Quickly stir in baking soda thoroughly.

Pour mixture into ungreased 13x9x2 inch pan. Do not spread! Cool and break into pieces. Can then be dipped in chocolate coating if desired.