Combine the sugar, syrup, and vanilla in a medium-size saucepan. Stir over medium heat until the sugar is dissolved.
Continue to cook to 300 degrees F, on a candy thermometer or hard crack stage, about 10 minutes. Cook's Note: You can use a candy thermometer or a simple trick: take a spoon and drop a tiny drizzle of the mixture into a bowl of water. If it crystallizes, you've reached the hard crack stage.
Remove from the heat, using a wooden spoon, stir in the baking soda quickly. It will become frothy. Pour into an 8 by 8-inch/20 by 20-cm pan, lined with parchment paper and buttered.
Let cool for about 30 minutes and break into pieces.
NotesChocolate Dipped Sponge Toffee: Dip the sponge toffee in chocolate for a homemade crunchy treat.
Sponge Toffee Ice Cream: Mix sponge toffee pieces into prepared ice cream.