Preheat oven to 375 degrees.
Lightly grease 15 x 10 x 1 inch jelly roll pan.
Line bottom of pan evenly with soda crackers.
Combine butter and brown sugar in 2 quart heavy saucepan.
Cook over medium heat, stirring constantly, until mixture comes to a boil.
Continue cooking 3 more minutes, stirring constantly.
Pour mixture evenly over crackers.
Bake for 15 minutes. Remove from oven.
Sprinkle with chocolate pieces. Let stand 5 minutes.
Spread melted chocolate over crackers.
While still warm, cut between crackers making 35 squares.
Chill in refrigerator until set.