The recipe for matzoh crunch/matzoh brittle (or matzoh crack as some rightfully call it) is all over the internet, but this site purports to be the originator:
http://www.betterbaking.com/viewRecipe.php?recipe_id=978
I use semisweet chocolate and sprinkle toasted slivered almonds over the melted chocolate. Because I have no patience, I throw the pan in the freezer to cool/solidify the matzoh crunch quickly, then break it up into bite-size pieces. Easy peasy recipe, and can be made well in advance (so long as you have the discipline not to devour it all before the seder even starts). Just make sure you don’t let the caramel burn in the oven.
And as long as I’m linking, these macaroons are very tasty if you’re a coconut lover–nothing at all like the canned Manischewitz stuff:
http://www.marthastewart.com/314385/chocolate-chunk-macaroons?autonomy_kw=macaroons&rsc=header_7
I find that most Passover cake/brownie recipes end up being disappointing, so I just avoid them. The recipe below has been sitting in my bookmarks for years, but I’ve never gotten around to trying it since we have enough chocolate from the matzoh crunch, but it sounds good and has a ton of positive reviews (even though it looks pretty grim in the photograph). I like that it doesn’t use any flour substitutes, like matzoh cake meal, which I think are what make a lot of Passover dessert recipes problematic.
http://allrecipes.com/recipe/16291/passover-chocolate-torte/
My other seder must-haves (recipes available if anyone is interested) are Ellyn Goodrich’s gefilte fish terrine that even gefilte fish haters think is great–it’s a pain to make, because you have to grind the fish in the food processor, kind of a yucky experience (the little fish store that used to do it for me closed down–sigh); and Passover rolls from a recipe that was in a collection published by the local B’nai B’rith which I received as a bridal shower present 42 years ago–they are all eggs, matzoh meal and Crisco and could be the most unhealthy thing I make all year, but they are beloved at our table.