Preheat the oven to 400°. In a food processor, combine the flours, sugar and 1/2 teaspoon of salt and pulse for 15 seconds. Add the butter and pulse for another 15 seconds, until a coarse meal forms. Add 1/2 cup of milk and pulse until the dough just comes together, about 30 seconds; add the extra 1/4 cup of milk if necessary. Gather the dough into a ball and wrap in plastic. Refrigerate for 15 minutes.