Preheat oven to 375 degrees F. Spray an 8-inch square pan with nonstick baking spray.
In a medium bowl, mix together first five ingredients (flour, Splenda, salt, and baking powder). Cut in margarine or butter until mixture resembles coarse crumbs. Sprinkle buttermilk over mix and blend.
Press onto the bottom of the prepared pan. Chill for 15 minutes and then bake for 15-20 minutes or until lightly browned.
While crust is baking, prepare topping. In a large bowl, beat eggs and egg white with 3 Tbsp flour and 1/2 cup of Splenda or Splenda/Sugar blend. Beat in 1/3 cup buttermilk, lemon juice, and zest. Pour over hot crust.
Turn oven down to 350 degrees F, and bake bars an additional 18-20 minutes or until top is set. Cool completely.
Just prior to serving, dust with powdered sugar if desired.