1. Combine first 3 ingredients in a food processor. Add 3/4 cup butter pieces to food processor, and pulse 12 to 15 times or until mixture resembles coarse meal; chill remaining 1/4 cup butter pieces. Add egg yolks, and process just until mixture forms clumps. (Clumps will be moist.) Press mixture onto bottom of a lightly greased 13- x 9-inch pan.
2. Bake at 350° for 16 to 18 minutes or just until golden brown around edges. Remove pan from oven, and reduce temperature to 325°.
3. Whisk together eggs and 2 cups sugar in a large bowl until blended. Combine 1/2 cup flour and baking powder; whisk into egg mixture until blended. Whisk in 4 tsp. grated lemon rind and 1/2 cup fresh lemon juice. Immediately pour lemon mixture over hot crust in pan.
4. Bake at 325° for 20 to 25 minutes or until filling is set. Remove from oven.
5. Combine 1/4 tsp. salt, remaining 1/2 cup sugar, and remaining 3/4 cup flour. Cut in reserved 1/4 cup butter pieces with a pastry blender or fork until crumbly. Sprinkle topping over hot lemon mixture, and bake 25 more minutes or just until lightly golden. Let cool completely on a wire rack. Cut into 32 (1 1/2- x 2-inch) bars, and sprinkle evenly with powdered sugar.