Pour mixture over crackers and smooth quickly and evenly using an offset spatula. Bake until toffee darkens and bubbles, 13 to 15 minutes. Transfer to a wire rack. Sprinkle chocolate evenly over toffee; let stand 2 to 3 minutes to soften. Using a clean spatula, smooth chocolate over toffee. If desired, sprinkle with nuts and lightly sprinkle with flaky sea salt. Cool completely. Break into pieces.