Line a large (or two smaller) cookie sheets completely with foil.
Cover the bottom of the sheet with baking parchment on top of the foil.
(This is very important since the mixture becomes sticky during baking.) Line the bottom of the cookie sheet evenly with the matzo, cutting extra pieces, as required, to fit any spaces.
In a 3-quart, heavy-bottomed saucepan, combine the butter or margarine and the brown sugar.
Cook over medium heat, stirring constantly, until the mixture comes to a boil, about 2 to 4 minutes.
Boil for 3 minutes, stirring constantly.
Remove from the heat and pour over the matzo, covering completely.
Place the baking sheet in the oven and immediately reduce the heat to 350°F.
Bake for 15 minutes, checking every few minutes to make sure the mixture is not burning.
If it seems to be browning too quickly, remove the pan from the oven, lower the heat to 325°F, and replace the pan.
Remove from the oven and sprinkle immediately with the chopped chocolate or chips.
Let stand for 5 minutes, and then spread the melted chocolate over the matzo.
While still warm, break into squares or odd shapes.
Chill, still in the pan, in the freezer until set.
(It wasn't completely necessary to cut or break the pieces apart until after it was good and solid.) As a variation, use"coarsely chopped white chocolate or a combination of white and dark, and chopped or slivered toasted almonds, sprinkled on top as the chocolate sets.
You can also omit the chocolate for a caramel-alone butter crunch.