Start by lining a baking sheet with parchment paper, making sure it hangs over the sides slightly. Arrange the matzo pieces to cover the baking sheet, breaking them to fit if needed.
In a large saucepan, combine the butter, sugar, water, corn syrup (or agave), and a few pinches of kosher salt. Heat the mixture over medium heat, stirring gently until the butter melts and a smooth mixture forms. Scrape down any crystals that form on the sides of the pan. Continue cooking until the mixture turns caramel in color and reaches 300°F, which should take about 10 to 15 minutes. Keep a close eye on it to prevent burning.
Once the caramel reaches the right temperature, remove it from the heat immediately and pour it over the matzo on the baking sheet. Let it sit for about 30 seconds.
Sprinkle the caramel with chocolate chips and allow them to sit for another 30 seconds to a minute, until the chocolate begins to melt. Use a spatula or the back of a spoon to spread the chocolate evenly over the caramel, covering it completely.
Sprinkle the melted chocolate with crushed walnuts, flaky sea salt, and any other toppings you like. Gently press the toppings into the chocolate.
Transfer the baking sheet to the fridge and let the candy cool and set for about 1 hour.
Once the chocolate toffee matzo has cooled and hardened, remove it from the fridge and break it into smaller pieces using your hands or a knife.
Serve immediately, or store the candy in an airtight container in the fridge for up to 3 months.