1. Butter 15-1/2x10-1/2x1-inch jelly-roll pan.* Sprinkle 1 cup peanut butter chips evenly onto bottom of prepared pan; set aside.
2. Melt butter in heavy 2-1/2-quart saucepan; stir in sugar, corn syrup and water. Cook over medium heat, stirring constantly until mixture reaches 300°F. on candy thermometer; this should take 30 to 35 minutes. (Bulb of thermometer should not rest on bottom of saucepan.)
3. Remove from heat. Immediately spread mixture in prepared pan; sprinkle with remaining 2/3 cup peanut butter chips. Cool completely. Remove from pan. Break into pieces. Store in tightly covered container in cool, dry place. About 2 pounds brittle.
* For thicker brittle, use a 13x9x2-inch baking pan.