Have ready a marble slab or a large baking sheet lined with buttered parchment paper or a silicone liner and set on a wire rack. Combine in a large heavy saucepan:

Stir over medium heat until the sugar is dissolved. Wash down the sides of the pan with a pastry brush dipped in warm water. Increase the heat to medium-high, bring the mixture to a boil, and cook, without stirring, to 330°F, the light-caramel stage.

Remove from the heat and quickly stir in:

Carefully pour the mixture out onto the slab or baking sheet. Place a second silicone liner or piece of generously buttered parchment paper on top of the brittle. Cover your hands with oven mitts and press the brittle into a thin, even layer. Let cool completely. Break the brittle into pieces. Store between layers of wax or parchment paper in an airtight container at room temperature for up to 1 month.