Line a rimmed 9 x 13-inch baking sheet with parchment paper or a silicone baking mat. Lightly grease the liner if needed so the set candy releases easily.

Place the butter, sugar, light corn syrup, water, and salt, if using unsalted butter, in a heavy-bottomed saucepan. Set the pan over medium heat and stir frequently until the butter melts and the mixture begins to boil.

Continue cooking over steady medium heat, stirring consistently, until the candy thermometer reads 300°F (149°C). This hard-crack stage generally takes 12 to 18 minutes, and the mixture should be deep golden brown.

Remove the saucepan from the heat and quickly stir in the vanilla and about half of the chopped almonds. Use caution because the hot toffee may bubble vigorously.

Immediately pour the hot toffee onto the prepared baking sheet. Spread it into an even layer while it is still fluid, working quickly without touching the candy directly.

Sprinkle the chocolate chips evenly over the hot toffee. Let them stand for 2 to 3 minutes so they soften, then spread the melted chocolate across the surface.

Scatter the remaining chopped almonds over the melted chocolate. Gently press them into the chocolate so they stay attached once the candy firms.

Allow the Almond Roca to cool completely at room temperature, or refrigerate it until firm for about 1 hour. Do not try to break it while the center is warm, because the layers can separate.

Break the set slab into irregular pieces or cut it into neat squares with a sturdy knife. For a traditional chocolate-coated version, turn the first set slab over, spread melted chocolate on the underside, add almonds, and let it set before breaking.