01 - Preheat oven to 350°F. Line a rimmed baking sheet with parchment paper or foil. Arrange matzo sheets in a single layer to cover the pan, breaking as needed to fit.
02 - In a saucepan, melt butter over medium heat. Add brown sugar and salt; stir constantly until mixture comes to a boil. Continue boiling for 3 minutes, stirring frequently, until thickened and bubbling.
03 - Remove from heat and stir in vanilla. Immediately pour the hot toffee evenly over the matzo, spreading with a spatula to cover all surfaces.
04 - Transfer baking sheet to oven and bake for 10 minutes, or until toffee is bubbling and darkened.
05 - Remove from oven and, while hot, sprinkle chocolate chips evenly over the toffee layer. Let sit for 3 minutes to allow chocolate to soften, then spread chocolate into a smooth, even layer.
06 - Sprinkle with chopped nuts (if using) and flaky sea salt.
07 - Let cool at room temperature for 30 minutes, then refrigerate for at least 30 minutes until set. Break into pieces and serve.