Position rack in middle of oven and preheat to 425°F. Butter loaf pan.

In small saucepan over moderate heat, heat butter until hot but not smoking. Stir in 3 tablespoons sesame seeds and sauté until seeds are golden brown and fragrant, about 4 minutes. Transfer mixture to small bowl and let cool to room temperature.

In small bowl, sprinkle gelatin over 2 tablespoons cold water. Stir, then let stand until softened, about 5 minutes.

In large bowl, stir together sugar and 1/2 cup warm water (105°F to 115°F). Sprinkle yeast over and let stand until foamy, about 5 minutes. Add 2 eggs, buttermilk, molasses, butter–sesame seed mixture, and gelatin mixture, and whisk to combine.

In large bowl of electric mixer fitted with paddle attachment, whisk together tapioca flour, chickpea flour, almond flour, coconut flour, amaranth flour, sorghum flour, cornmeal, potato flour, cornstarch, xantham gum, and salt. Add wet ingredients and beat at moderate speed until dough is aerated and holds its shape, about 4 minutes. Scrape down bowl, then beat at high speed for 1 minute. Transfer dough to prepared pan and smooth top with rubber spatula. (If necessary, use a wet hand to smooth completely.) Cover loosely with plastic wrap and let rise in warm place until dough is just level with top of pan, about 1 hour.

In small bowl, whisk together remaining egg and 1 teaspoon water. When dough has risen, lightly brush egg wash over top, then sprinkle with remaining 1 tablespoon sesame seeds.

Bake bread until firm, about 30 minutes, then carefully turn loaf out of pan and continue baking directly on oven rack until bottom sounds hollow when tapped, about 15 minutes more. Transfer to wire rack and cool at least 1 1/2 hours before slicing. (Store bread, wrapped in aluminum foil, 3 days at room temperature, 1 week refrigerated, or 1 month frozen.)