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.

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

Remove the plastic wrap and, using a sharp knife or lame, slash the top of the loaf from one short end to the other about 1/4-inch deep.

Brush the top of the risen loaf generously with melted butter before baking as an optional step to aid the loaf in browning.

Place the pan in the center of the preheated oven and allow to bake for 30 minutes.

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).

Remove the pan from the oven and allow the bread to cool for about 10 minutes in the pan before transferring to a wire rack to cool completely before slicing and serving.

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.