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Weigh your flour; or measure it by gently spooning it into a cup, then sweeping off any excess. Place the flour, sugar, yeast, salt, and xanthan gum in a bowl or the bowl of your stand mixer. Mix until combined.
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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.
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Add the sourdough starter and butter, and beat until thoroughly blended.
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Beat in the eggs one at a time, mixing 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.
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Cover the bowl and let the batter rise for about 90 minutes, until it's expanded in size and fluffy.
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Scrape down the bottom and sides of the bowl, gently deflating the batter in the process.
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Grease an 8 1/2" by 4 1/2" loaf pan; or a 9" x 4" x 4" gluten-free loaf pan, which will give you a taller, more shapely loaf.
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Scoop the dough into the pan. Press it level, using a spatula or your wet fingers.
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Cover the pan with something that leaves room for expansion; a bowl cover positioned so it's not flat across the top of the pan works well here. Set the bread in a warm place to rise until it 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" pan. This should take about 60 minutes. Toward the end of the rising time, preheat the oven to 350°F.
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Bake the bread for 38 to 42 minutes, until golden brown. Remove the bread from the oven, turn it out of the pan, and cool on a rack.
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Store leftover bread, well wrapped, at room temperature for several days; freeze for longer storage.
