Preheat oven to 325 degrees F. Line a 9x9x2-inch baking pan with foil, extending foil over edges of the pan. Grease foil; set aside.
For crust, in a medium bowl, stir together flour, baking powder, and salt. In a large bowl beat butter with an electric mixer on medium to high speed for 30 seconds. Add sugar. Beat for 5 minutes, scraping bowl occasionally. Add egg, beating well. Gradually add flour mixture, beating on low speed until combined. Spread evenly in prepared pan. Bake 20 minutes or until crust is lightly browned and feels nearly firm in the center. Cool slightly on wire rack.
In a small saucepan melt white chocolate over low heat. Spread evenly over the crust. Let stand until set (chill briefly if needed).
Use foil to lift uncut bars out of pan. Spread with dulce de leche. Cut into bars. Top with peaches; sprinkle with pistachios. Makes 16 bars.