Preheat oven to 350 degrees Fahrenheit. Grease 13"x9" metal baking pan. Line pan with foil; grease foil.
In a heavy 3-quart saucepan, melt margarine or butter over low heat. Remove saucepan from heat. With wooden spoon, stir in brown sugar and vanilla, then stir in eggs until well blended. In a small bowl, combine flour, baking powder, and salt; stir into sugar mixture just until blended. Stir in chopped pecans.
Spread batter evenly in pan. Bake 30 minutes or until toothpick inserted 2 inches from edge comes out clean. Do NOT overbake; blondies will become firm as they cool. Cool completely in pan on wire rack.
When cool, transfer with foil to cutting board. Cut blondies lengthwise into 4 strips, then cut each strip crosswise into 6 pieces.