Grease a standard 9-inch x 5-inch loaf pan and line with parchment paper. Set the pan aside.

In the bowl of a stand mixer fitted with the paddle attachment or a large bowl with a hand mixer fitted with dough hooks, place the all purpose flour, xanthan gum, tapioca starch/flour, granulated sugar, and salt, and mix or whisk to combine well.

Add the starter, 1 1/2 cups of milk, and the butter, and beat on medium speed to combine. This is a batter-style bread dough, so it won’t resemble traditional bread dough, but rather a soft cookie dough.

Turn the mixer up to medium-high speed and beat until the dough has taken on a whipped appearance (about 5 minutes). The dough should be tacky to the touch, but should hold its shape when scooped.

If your dough feels at all dry to the touch, add more milk by the tablespoon, beating it in until well-combined, until the dough reaches the proper consistency.

Transfer the bread dough to the prepared loaf pan. Using a moistened spatula, press the dough into every corner of the loaf pan and spread the top into an even layer.

For a more traditional loaf shape, pile the dough a bit more toward the center in a dome.

Cover the loaf pan with lightly oiled plastic wrap and allow it to rise in a warm, draft-free place until it’s reached about 150% of its original size, at least 4 hours. It will not fully double in volume, and will rise more in the oven than it does raw.

This bread dough is much less likely to overproof and take on that pock-marked appearance than bread made with conventional yeast. If you’re unsure about whether the bread has proofed enough, allow it to keep rising.

When the bread is nearing the end of its rise, preheat your oven to 400°F.

Reduce the oven temp to 350°F, rotate the pan 180° around, and continue to bake until center of the loaf reads 200°F on an instant read thermometer (about 30 minutes more).

To freeze the bread, cool the loaf completely, then slice, wrap tightly and freeze the slices. Defrost as many slices at a time as you need in the toaster.