Line a 9x13-inch pan with parchment and lightly butter it; sprinkle half the nuts evenly over bottom.
Combine butter, sugar, water, and salt in a heavy-bottomed saucepan; cook over medium, stirring constantly, until butter melts and sugar dissolves.
Attach candy thermometer to pan; cook 12-15 minutes, stirring frequently, to 300°F (hard crack) and golden amber color.
Immediately remove from heat; stir in vanilla and carefully pour hot toffee over nuts, spreading quickly with a buttered spatula.
Cool 1-2 minutes; sprinkle chips evenly over hot toffee. Wait 2-3 minutes to soften, then spread chocolate evenly with an offset spatula.
Immediately sprinkle remaining nuts over chocolate; press gently to adhere.
Refrigerate at least 1 hour until completely set; break by hand or cut into squares with a sharp knife.
Store airtight with parchment between layers up to 3 weeks at room temperature or 1 month refrigerated.