Combine dry ingredients in a mixer and blend well. Beat in butter until dough forms and pulls away from sides of bowl (beat on high for at least 3 minutes).

Place a 9-9 1/2" spring form pan (without its bottom) onto a parchment or silcon lined baking pan. Press dough into pan, spreading out into an even flat layer. Take a 2" cookie cutter and stamp out center. This will allow the center to bake at the same rate as the outsides. Place cutter back into center to keep the cookies shape while baking.

Place on center rack in a 450°F preheated oven. Immediately decrease to 300°F and bake for 20 minutes, just until edges begin to brown. Turn oven off.

Remove spring form collar and lightly score the shortbread into 16 wedges. Using a skewer, poke about 8 holes in each wedge and return shortbread pan to oven. Keep oven door slightly ajar by propping it open with a wooden spoon (so there is about 1" opening). Allow to sit in cooling oven for an hour.

Remove from oven and allow to cool completely at room temperature before cutting into wedges.