Preheat oven to 350°. Line a 15×10-inch baking pan with foil. Coat foil with cooking spray. Arrange crackers snugly in rows to cover the entire pan, breaking crackers in half if necessary to fill any gaps.Advertisement
In a medium saucepan, melt butter over medium-high heat. Add brown sugar and fine sea salt. Bring to a boil, stirring often. Cook 3 minutes, stirring occasionally. Remove from heat. Stir in vanilla.
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.