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Spread half of the almonds over 1 very large (12- by 17-inch) baking sheet (or 2 medium baking sheets).
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Combine the espresso powder and vanilla in a small bowl and set it next to the stove.
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Combine the butter, sugar, and water, and salt in a 3-quart saucepan with a heavy bottom. Set the saucepan over medium heat and stir occasionally until the sugar dissolves and the mixture comes to a boil.
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Once the mixture boils, insert a candy thermometer and stir constantly until the candy reaches 300 degrees.
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Remove pan from the heat and quickly stir in the instant espresso powder and vanilla. CAREFULLY pour the mixture over the almonds.
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Scatter the chopped chocolate over the toffee and let it sit for 2 minutes. Use an offset metal spatula to spread the chocolate into a thin layer and sprinkle the remaining almonds on top.
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Cool the toffee at room temperature for 4 hours, until the toffee and chocolate are both set. (Depending on the temperature in your kitchen, you may need to refrigerate the toffee once it’s cooled in order to set the chocolate.) Break into pieces and serve. Store in an airtight container in the fridge for up to 1 week or the freezer for up to 3 months.