Preheat oven to 400 °F.
Butter a 12-cup muffin tin, or line it with paper liners.
In a medium-size bowl, beat egg whites until stiff but not dry.
Sift flours and baking powder together 3 times.
Place into a large bowl.
In another medium-size bowl, beat together egg yolks, buttermilk, maple syrup, and butter until blended.
Pour liquid ingredients into dry ingredients, stirring until just blended.
Stir in walnuts.
Fold in egg whites with a spoon.
(Batter is too heavy for a wire whisk.) Divide batter among muffin cups; they will be almost full.
Bake on top shelf of oven for 25 minutes, or until lightly browned on top and dry inside when pierced with a cake tester or food pick.