Step 1Heat the oven to 350 degrees. Line a muffin tin with paper baking cups.
Step 2Heat the milk in a small saucepan to just under boiling. Remove the pan from the heat and stir in the poppy seeds. Let the milk cool until it's just warm.
Step 3Beat the butter with the sugar until light and fluffy. Beat in the egg yolks. Sift the all-purpose and whole-wheat flours with the baking powder and salt and add alternately with the milk and poppy seeds to the butter mixture, beginning and ending with the flour.
Step 4Beat the egg whites just until they're stiff and gently fold them into the batter. Evenly divide the batter among the cups, filling them about 3/4 full. Bake the muffins until a toothpick inserted in the middle comes out clean, 25 minutes.