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In a large, deep (3 quart) saucepan, melt the butter. Stir in the salt, sugar, espresso powder, water, and corn syrup, and bring the mixture to a boil. Boil gently over medium heat, without stirring, until the mixture reaches hard-crack stage (300°F on an instant-read or candy thermometer; you'll be taking it off the heat a few degrees sooner). The syrup will bubble without seeming to change much for awhile, but be patient; all of a sudden it will darken, and at that point you need to take its temperature and see if it's ready. (If you don't have a thermometer, test a dollop in ice water; it should immediately harden to a brittleness sufficient that you'll be able to snap it in two, without any bending or softness). This whole process should take about 10 to 12 minutes. Pay attention; too long on the heat, and the syrup will burn.
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While the sugar is boiling, spread half of the nuts in an even, closely packed layer on a parchment-lined 9" x 13" pan. Top the nuts with half the chocolate.
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When the syrup has reached 295°F, remove from the heat and stir in the baking soda (if using); it will foam up, so use caution. Pour the syrup quickly and evenly over the nuts and chocolate. Top with the remaining chocolate and let sit for 2 to 3 minutes, until it softens; spread the chocolate with an offset spatula in an even layer and immediately sprinkle the remaining nuts on top.
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While the candy is still slightly warm, pull it out of the pan and use a thin spatula to loosen it from the parchment. When completely cool, break into uneven chunks.
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Store cooled candy tightly wrapped; it'll stay fresh for a couple of weeks at room temperature. Freeze for longer storage.
