Dark Chocolate Buttercrunch

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., 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., When the syrup has reached 295°F, remove from the heat and stir in the baking soda; 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., 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., Store cooled candy tightly wrapped; it'll stay fresh for a couple of weeks at room temperature. Freeze for longer storage.