1Heat oven to 375ºF. Grease rectangular pan, 13x9x2 inches. Heat butter and brown sugar in 3-quart saucepan over low heat about 5 minutes, stirring occasionally, until butter is melted; remove from heat.
2Beat vanilla and eggs into sugar mixture with electric mixer on medium speed. Beat in flour and baking powder on low speed. Stir in half of the nuts and 1/4 cup of the baking chips. Pour into pan. Sprinkle with remaining nuts and 1/2 cup of the baking chips.
3Bake 25 to 30 minutes or until top is golden brown and center is set. Cool about 30 minutes.
4Melt remaining 1/4 cup baking chips and the shortening in 1-quart saucepan over low heat, stirring constantly. Pour into small resealable plastic food-storage bag; seal bag. Cut small opening in a corner of the bag. Squeeze bag to drizzle melted chips over bars. Cool until set. For 48 bars, cut into 8 rows by 6 rows.