Instructions:
- Prepare the pan: Line an 8x8 inch pan with parchment paper, ensuring an overhang on two sides for easy lifting later. Lightly grease the paper.
- Melt Butter Base: In a heavy-bottomed 3-quart saucepan, combine the butter, sugar, water, corn syrup, and salt over medium heat. Stir gently until the butter is melted and the sugar is completely dissolved (no graininess remains).
- Wash Down Sides: Using a damp pastry brush, carefully wipe down any sugar crystals clinging to the sides of the pan to prevent premature crystallization.
- Cook to Temperature: Insert the candy thermometer. Bring the mixture to a boil, stopping all stirring. Cook undisturbed until the temperature reaches precisely 300°F (149°C)—the hard crack stage. This usually takes 15-20 minutes.
- Pour and Cool: Immediately remove the pan from the heat. Carefully pour the hot mixture into the prepared pan. Do not scrape the pan. Let it cool completely at room temperature until hard (about 1 hour).
- Prepare Chocolate: Once the toffee is cool and hard, melt the dark chocolate using a double boiler or microwave in short bursts, stirring until smooth and glossy.
- Coat and Sprinkle: Spread the melted chocolate evenly over the cooled toffee slab. Immediately sprinkle with toasted almonds, if using.
- Set and Break: Allow the chocolate to set completely (refrigeration for 30 minutes can speed this up). Once fully set, use the parchment overhang to lift the slab and break it into uneven, rustic pieces.