Step 1 Cut the butter into medium-sized cubes and put it in the freezer. Put half a cup of cold water in the freezer too while you assemble the other ingredients for the recipe. Put 2 sheets of plastic wrap, about 24 inches long, on a work surface, overlapping them lengthwise by a few inches so that they form a big triangle.
Step 2 Put the flours, sugar and salt in the bowl of a food processor, then pulse it once or twice to make sure the ingredients are combined. In a small bowl, mix together the egg yolk and vinegar. When the butter is quite hard, after about one-half hour, add the pieces to the food processor. Pulse it half a dozen times, until the pieces of butter are about the size of jelly beans.
Step 3Take the ice water out of the freezer (it should be very cold but not frozen) and mix it into the egg and vinegar mixture. Immediately add this to the mixture in the food processor and pulse until the mixture begins to come together. The bits of butter will still be visible, about the size of peas.
Step 4Dump the mixture out into the middle of the plastic wrap. Pull the edges of the plastic up around the dough, squashing it together and blending in the streaks of butter, which you should still be able to see in the dough. Don't overwork the dough or blend in all the butter: it should be streaky and just come together.
Step 5Using the plastic to press the dough, form it into a rectangle, wrapping the plastic tightly around the dough in the process. Put the dough in the refrigerator to chill for at least an hour. (You can keep it in the refrigerator for a few days and up to a month in the freezer.)