In large bowl - Sift flours and extra fine sugar together FOUR times.
Use pastry blender or fork to cut softened butter into the flour / sugar mixture until crumbly.
Use a bread hook on mixer to blend ingredients, at #2 speed, for 10 to 15 minutes.
Line 10x15 inch cookie sheet with sides with tin foil, leave enough edges so you can lift the whole thing out later.
Dump mixture into lined pan and pat to distribute evenly. This will be very crumbly. Use hands to pat together, then roll top of surface and try to get dough at an even thickness all across the pan.
Refrigerate pan and dough 1-2 hours (1 hour is usually enough). Properly chilled dough should not crack when you begin to cut into bars.
Preheat oven to 275 degrees.
Lift dough out of pan and begin to cut into 1x2" bars (they will expand while baking). Note: I use a pizza wheel / cutter tool and it works very well. Just place a straight edge ruler on top of the dough then cut slowly and gently rolling the wheel back and forth until you cut all the way through.
Prick each bar with fork across the face of the cookie about 4 times.
Place bars on cookie sheet (I use a silpat mat and don't have to grease pan). Place 1 to 2 inches apart. Keep the rest of the dough chilled while baking cookies.
Bake 40 minutes or a bit longer if you like the bottoms to be slightly brown. Tops will not be browned at all. Remove and place on wire rack to cool.
When cookies are cool roll in additional Baker's sugar to coat. Store in air tight container.
These cookies travel well if boxed and layered between parchment paper, and they freeze well too.