Sprinkle one package of chocolate chips on prepared pan and place in warm oven until chips melt.
Remove from oven, spread melted chocolate over bottom of pan; set aside.
In a large heavy saucepan over medium-high heat, combine butter and brown sugar. Stirring constantly, heat to 300 to 310 degrees Fon a candy themometer, or until a small amount of syrup dropped into cold water forms hard, brittle threads. Immediately remove from heat.
Stir in 3/4 cup slivered almonds.
Pour almond mixture onto pan with melted chocolate; spread mixture evenly.
Sprinkle remaining package of chocolate chips over the almond layer and spread chocolate evenly once melted.
Sprinkle remaining almonds over chocolate.
Cut into squares, or allow to harden in a solid sheet and break it apart like brittle.