Stir the flour, sugar, yeast, salt, and xanthan gum together in your mixer bowl.
With the flat beater running on low, stream in the warm milk. It'll look ragged and wrong for a minute. Keep going.
Beat in the butter until it vanishes into the mix.
Add the eggs one at a time, letting each disappear before the next goes in.
Bump the mixer to HIGH and run it a full 3 minutes. This is the step doing gluten's job — don't shortcut it. You'll end up with something closer to frosting than dough. That's right.
Cover and set somewhere warm for about an hour. It'll get puffy, not doubled. That's correct.
Deflate gently, then scrape into a greased 8 ½ x 4 ½ loaf pan.
Cover loosely, let rise until it just crowns the rim (about 30 more minutes). Heat your oven to 350°F as it finishes.
Bake 38 to 42 minutes, until deep gold on top and about 205°F inside.
Turn out onto a rack. Rub the warm top with a stick of butter if you want that soft, glossy crust.
Cool completely before you slice. Non-negotiable.