Preheat oven to 350º
Put wheat bran in large measuring cup and pour boiling water over it, stir to combine and let sit 5 minutes, then stir in the Greek yogurt and eggs, mix well.
Meanwhile, place all dry ingredients in a bowl ~ flours, sugar, baking powder, baking soda, salt, cinnamon, nutmeg ~ and whisk to combine.
Transfer dry mixed ingredients to bowl of stand mixer and add the really soft butter. Mix on low speed until butter is fully incorporated.
Add the bran mixture and mix until just combined. Remove bowl from stand and with spatula, make sure batter is fully mixed in from the bottom.
Evenly fill well-buttered muffin tin with batter, each of the 12 cups will be pretty full. The batter rises and puffs but does not spread. Bake in center of oven until firm, about 18 - 22 minutes. Pop muffins out to wire rack and serve warm with lots of butter. Store remaining muffins for 2 days or freeze.