"Whole-grain flours lend these popovers an earthy nuttiness."
Heat oven to 450 degrees. Brush cups of a popover pan (or muffin tin) with butter or coat with cooking spray.
In a measuring pitcher with a spout (this makes pouring easier later), or in a bowl, whisk together milk, eggs, butter, sugar and salt until frothy. Add flours and whsk until mostly smooth, though a few clumps may remain in batter, which is fine. (If you prefer you can mix everything together in a blender instead of a bowl).
Pour batter into prepared cups. Note that this recipe yields 6 popovers (I filled all twelve of the muffin tin cups for smaller popovers). Bake 20 minutes. Turn heat down to 350 degrees and bake another 20 minutes until popovers are golden brown and puffed. (Reduce baking time by 5 minutes if using a muffin tin.) Keep tabs on their progress by looking through the window in the oven door. Do not open the oven door until the last 5 minutes of baking or they won't puff. Serve warm.