Preheat oven to 425 degrees. Move an oven rack to the middle position.
Place a circle of parchment paper in a 9-inch tart pan with removable bottom, or in a round cake pan.
In the bowl of an electric mixer, blend the flours, 2/3 cup sugar and salt.
Cut the butter into ½-inch cubes and add to the flour mixture. Mix at low speed for several minutes until the mixture resembles damp crumbs. Do not overmix.
Pour half the mixture into the prepared pan, smooth into an even layer, then firmly press with a juice glass or back of a spoon. Add the rest of the mixture and repeat, pressing firmly.
If using a cake pan, place a baking sheet on top of the pan, then invert. Tap the bottom of the pan until the dough releases, then carefully lift off pan and remove parchment. If the dough cracks a bit, press back together. (If it crumbles a lot, repeat the whole process; for this reason, a tart pan is preferred.)