1 cupcoarsely chopped walnuts, toasted if desired (optional)
Softened unsalted butter, for greasing the pan
Set a rack in the center of the oven and preheat to 400°F.
In a large bowl, whisk together the all purpose flour, whole wheat flour, baking powder and salt. Set aside.
In a medium bowl, whisk together the maple syrup and melted butter. Whisk in the milk, followed by the egg and egg yolk. Add to the dry ingredients and whisk until just smooth. Stir in the walnuts, if using. Let the batter rest for 5 minutes.
Meanwhile, grease the muffin pan generously with softened butter (see my note about the muffin pan below).
Spoon the batter evenly into the prepared pan. The cups will be almost full. Bake for 10 minutes, then reduce the temperature to 375°F and bake for about 15 minutes more, or until the muffins are golden and a cake tester inserted into the center of a muffin comes out clean.
Cool in the pan for 5 minutes, then remove muffins from the pan. Serve warm with butter, or place on a rack to cool.
Note: My only quibble with this recipe is that the muffins tend to stick to the pan. I found that letting them cool for only 5 minutes (instead of the recommended 10) helped them release more easily. If they stick, take a butter knife or offset spatula and run it around the edges; and instead of pulling the muffins out of the pan, turn the pan over and knock it gently on the counter until the muffins drop out. Or, use paper liners and call it a day (but still butter the top of the pan!). I also found that the recipe is perfect for 12 muffin cups without the nuts; when you add the nuts, the muffins run into each other a bit, making them difficult to remove. So, if you're adding the nuts, you might want to bake the muffins in two pans, filling 6 cups in one pan and 7 in the other, for a total of 13 muffins.
Freezer-Friendly Instructions: The muffins can be frozen in an airtight container or sealable plastic bag for up to 3 months. Thaw for 3 – 4 hours on the countertop before serving. To reheat, wrap individual muffins in aluminum foil and place in a preheated 350°F oven until warm.