Line the bottom of the baking sheet with matzo or crackers, covering all parts in a single layer. [If using matzo, you’ll need to break pieces to fit any extra spaces, which will be annoying because despite being perforated, it does not actually break in straight lines. I have some luck making cuts by gently, carefully sawing with a serrated knife.]
Make the caramel/toffee: In a medium saucepan, melt the butter and brown sugar together, and whisk it over medium heat until it begins to boil. Once it has begun boiling, let it bubble for 3 more minutes, whisking it the whole time. The butter and sugar will come together, and the mixture will thicken a little as it cooks. Remove from the heat and add a couple good pinches of sea salt and vanilla, if using, and then quickly pour it over the matzo or crackers. Use an offset spatula to spread the caramel quickly over all the crackers, as it will begin to set as soon after it is poured.
Bake: The caramel-covered crackers for 13 to 15 minutes, watching carefully as it will bubble and the corners might darken too quickly and/or burn. You can reduce the heat if you see this happening.
Finish the crackers: Remove from oven and immediately sprinkle with chocolate chips. Let stand 5 minutes, and then use an offset spatula to spread them in an even layer over the caramel. If you’re using them, sprinkle the chocolate with toasted chopped nuts and/or sea salt. (The extra sea salt is great on matzo. On saltines, it’s really not necessary.)
Cool: Once completely cool — I speed this along in the freezer, impatient as should be expected in the face of caramel crack(ers) — break or cut it into pieces and place in an airtight container. When cleaning up, you’ll see a lot of messy crumbs and bits; sweep them all into a jar for the best ice cream topping that has ever existed, trust me.
To store: Keep both the jar of crumbs and the crack(ers) in the fridge; it won’t go bad at room temperature but it’s less sticky and messy when it’s cold. It should keep for a week but I’ve never seen it last that long.