Place the flour, sugar, yeast, salt, and xanthan gum in a bowl, or the bowl of your stand mixer. Mix till combined., Using an electric mixer (hand mixer, or stand), drizzle in the milk, beating all the time; the mixture will be crumbly at first, but once all the milk is added, it'll come together., Add the butter and beat until thoroughly blended., Beat in the eggs one at a time, beating each in thoroughly before adding the next. Scrape the bottom and sides of the bowl, then beat at high speed for 3 minutes, to make a very smooth, thick batter., Cover the bowl, and let the thick batter rise for 1 hour., Scrape down the bottom and sides of the bowl, gently deflating the batter in the process., Grease an 8 1/2" by 4 1/2" loaf pan, or a 9" x 4" x 4" pain de mie pan., Scoop the dough into the pan. Press it level, using a spatula or your wet fingers., Cover with greased plastic wrap, and set in a warm place to rise until the loaf barely crowns above the rim of the 8 1/2" x 4 1/2" pan; or until it comes to within about an inch of the rim of the 9" pain de mie pan. This should take about 45 to 60 minutes. Towards the end of the rising time, preheat the oven to 350°F., Bake the bread for 38 to 42 minutes, until golden brown. If you're using a pain de mie pan, leave the lid on the entire time. Remove the bread from the oven, turn it out of the pan, and cool on a rack., Like a baguette, this bread has a short shelf life. For best texture, reheat or toast after the first day.