Preheat oven to 350 F. Combine first 3 ingredients in food processor. Add 3/4 cup butter to food processor, and pulse 12 to 15 times or until mixture resembles coarse meal. Add egg yolks to food processor, and pres just until mixture forms clumps. ( Clumps will be moist) Press mixture onto bottom of lightly greased 13 x 9 - inch pan.
Bake at 350 F for 16 to 18 minutes or just until edges are golden brown.Remove from oven, and reduce oven temperature to 325 F.
Whisk together 6 eggs and 2 cups sugar in large bowl until blended. Combine 1/2 cup flower and baking powder ; whisk into egg mixture until blended. Whisk in lemon zest and lemon juice. Immediately pour lemon mixture over hot crust in pan.
Bake at 325 F for 20 to 25 minutes or until filling is set. Remove from oven.
Combine 1/4 tsp. salt and remaining 3/4 cup flower and 1/2 cup sugar. Cut in 1/4 cup butter pieces with a pastry blander or fork until crumbly. Sprinkle topping over hot lemon mixture.
Bake at 325 F for 25 minutes or just until lightly golden.Let cool completely on a wire rack ( about 1 hour). Cut into 32 bars, and sprinkle evenly with powdered sugar just before serving.