Preheat oven to 350°F with a rack in the center.
Line a rimmed half-sheet pan tightly with foil. Then lay a piece of parchment overtop to cover the surface of the pan.
Arrange the matzos in a single layer to completely cover the pan, breaking them up as necessary to fill in any gaps.
Place the butter and brown sugar into a three-quart pot. It helps if the pot is nice and heavy (to distribute heat evenly) and light-colored on the inside (so you can see what's happening with the color of the caramel). It will seem big at first, but caramel can bubble up a lot.
Place pot over medium heat and cook, stirring, until butter melts and sugar dissolves.
Stop stirring and continue cooking until mixture comes to a boil.
Boil for two minutes.
Remove from heat and carefully pour caramel over matzo, covering completely.
Bake for 15 minutes, rotating pan halfway through.
Remove from oven and immediately sprinkle with chocolate. Let rest for five minutes.
If you're using nuts and they're not already toasted, toast them in a dry pan over medium heat while the chocolate melts.
Spread melted chocolate evenly over surface. Sprinkle with nuts if using.
Let the chocolate set completely. You can pop it into the fridge or freezer (right on the pan) to speed the process if you like. This may cause the chocolate to dull a bit more than it would otherwise, but in my opinion that's no big deal when making a homey recipe like this one.
Break cooled bark into shards or squares.