Grease or line a 9-inch x 5-inch loaf pan, or a 1-pound Pullman-style loaf pan (with a lid) and set it aside. You can also line the bottom and sides of the pan with a piece of parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, place the flour blend, xanthan gum, instant yeast, cream of tartar and sugar. Whisk together with a separate, handheld whisk. Add the salt, and whisk again to combine.
Add the milk, butter, vinegar and egg whites, and mix with the paddle attachment on low speed. Scrape down the sides of the mixer bowl as necessary during mixing.
Turn the mixer to medium-high speed and mix for about 3 minutes. The dough will be thick, smooth and quite wet, and should become slightly whipped in appearance.
Scrape the dough into the prepared loaf pan. Using a wet spatula, smooth the top.
Cover the dough with lightly oiled plastic wrap (and the lid of a pullman pan on top if using), and allow it to rise in a warm, draft-free place for 30 to 45 minutes or until it’s about 150% of its original size. That means that it's about 50% bigger than when it started (it won't double).
The dough may take longer to rise properly in colder, drier weather and less time in warmer, more humid weather. Be patient!
When the dough has nearly reached the end of its rise, preheat the oven to 375°F. The dough is starting to overproof if the top of the loaf starts to break and craters start to appear.
Remove the lid if using, and the plastic wrap. Using a sharp knife or lame slash the top of the loaf about 1/4-inch deep. If using the optional seeds, brush the top of the risen bread gently with melted butter, and sprinkle with the seeds.
If using a Pullman pan, grease the inside of the lid of the pan, and slide it into place on top of the pan.
Place the pan on the middle rack OR top oven rack (whatever is appropriate for your oven) of the preheated oven.
Bake for 45 minutes to 1 hour. If using a Pullman pan, remove the lid after about 40 minutes of baking and return the pan to the oven to finish baking.
The loaf is done when the internal temperature of the bread reaches about 205°F on an instant-read thermometer and a toothpick inserted in the center comes out mostly clean (not wet).
The outside will form a thick, brown crust (thinner and less brown if you've used a pullman pan with a lid), and the loaf will sound somewhat hollow inside when you tap it firmly but gently with your forefinger.
Remove the loaf from the oven, and allow the bread to cool for about 10 minutes in the pan. Transfer the bread from the pan to a wire rack, remove any parchment paper, and allow the loaf to cool completely. Slice the cool loaf into 10 equal slices with a serrated knife.
Wrap the cooled loaf and any slices tightly in plastic wrap and store at room temperature for up to 2 days.
To freeze this bread, cool completely and then wrap tightly, and freeze. Defrost the whole loaf at room temperature, still wrapped.
You can also slice a cooled loaf and wrap each slice separately, and freeze. Defrost as many slices at a time as you need in the toaster.