Preheat oven to 350°. Butter 12 muffin cups (1/3-cup size) and set aside. Put sesame and poppy seeds in a pie pan and sunflower and pumpkin seeds in another. Bake seeds, stirring once, until golden, 6 to 8 minutes. Combine seeds in 1 pan and set aside.
Whirl dates with 1/2 cup water in a blender or food processor until very smooth. Scrape into a medium bowl and add bananas, yogurt, oil, vanilla, egg yolk, and 3/4 cup sugar. Whisk to blend, then set aside.
Stir flours in a large bowl with flaxseed meal, baking powder and soda, salt, and all but 2 tbsp. of the seeds.
Beat egg whites with cream of tartar in a bowl with a mixer on high speed until foamy. Gradually add remaining 1/4 cup sugar, beating until soft peaks form. Fold whites into banana mixture. Fold into flour mixture just until blended. Spoon batter into prepared muffin cups (they'll be very full). Sprinkle with remaining 2 tbsp. seeds.
Bake muffins until just firm to touch and edges are golden, 22 to 25 minutes. Loosen from pans with a small metal spatula. Serve warm or cool.