Line a rimmed baking sheet with foil, making sure it goes up the edges, and top with parchment paper.
Spray parchment or use your butter wrappers to lightly grease.
Cover entire bottom of baking sheet with a single layer of matzos, breaking to fit when necessary. (Note: Although matzo is perforated and looks like it would break along nice even lines, it doesn't necessarily break cleanly like you'd expect. This is the bread of affliction, people; it's not here to make your lives easier...)
At this point, start preheating your oven to 350 degrees and make your toffee layer:
Add butter, brown sugar, vanilla, and just a bit of sea salt to a medium saucepan.
Cook over medium heat and whisk constantly until mixture comes to a boil, then continue cooking and whisking for another 3-4 minutes until toffee mixture is thickened, begins to get foamy, and pulls away from the sides of the pan as you stir.
Turn off your burner, pour toffee over the matzo layer, then spread evenly with a small rubber spatula. (Caution: This is approximately the temperature of molten lava when you first pour it, yet will harden quickly and become very difficult to spread. Get this layer done immediately, carefully, and quickly.)
Now bake your matzo-toffee concoction at 350 degrees for 10 minutes.
Your toffee layer should be bubbling all over when ready, and be careful not to overcook.
Remove from oven and immediately pour chocolate chips over the top. (I used Enjoy Life mini chips because that's what I had handy, but any semi-sweet chips should do.)
Let these sit for a few minutes until softened and melting, then use your spatula to spread the chocolate layer evenly across the top.
Top with chopped walnuts and a little sea salt.
Refrigerate for about 40 minutes, placing the hot cookie sheet on oven mitts or a towel to avoid the chance of cracking a glass refrigerator shelf.
When chocolate is hardened, lift the whole Matzo Crack out of the pan using the foil edges and break into pieces.
Store leftovers (ha! ha!) refrigerated in an airtight container.
If you are making Matzo Crack for Passover, leave out the vanilla extract (or use a substitute) and ensure that the brown sugar and chocolate you use are labeled Kosher for Passover. I also used unsalted matzo since that's traditional and the sale item we have here this week -- if yours happens to be salted; you might want to use unsalted butter.
Matzo Crack from Mashup Mom: https://www.mashupmom.com/matzo-crack/