Preheat the oven to 375°. Line a 9-by-13-inch metal pan with foil, leaving some overhang. Grease the foil. Line the bottom of the pan with crackers, placing them side by side and breaking to fit.
In a small saucepan over medium heat, bring the butter and sugar to a boil, stirring until the sugar dissolves. Boil for 4 minutes, stirring constantly. Remove from heat and stir in the espresso powder. Pour the mixture over the crackers, spreading to cover evenly.
Bake the toffee until the top bubbles vigorously, about 10 minutes. Dollop the peanut butter on top, then bake until it softens, about 1 minute. Remove from the oven. Using an offset spatula or the back of a spoon, spread the peanut butter evenly. Sprinkle with the chocolate and nuts. Cool to room temperature, about 30 minutes. Refrigerate uncovered until set, about 2 hours.
Using the foil overhang as an aid, transfer the toffee to a cutting board. Cut into squares. Refrigerate the toffee in an airtight container.