Step 1

In a small saucepan, place the roux ingredients and whisk to combine well. Cook the roux over medium-high heat, whisking constantly, until thickened enough that the whisk leaves a visible trail.

Step 2

Remove it from the heat immediately and allow to cool to at least warm room temperature. The roux can be made ahead of time and stored in a sealed container in the refrigerator. Just allow it to come to room temperature before baking with it.

Step 3

Grease and line a standard 9-inch x 5-inch loaf pan and set it aside.

Step 4

In the bowl of a stand mixer fitted with the paddle attachment, place 3 cups of the flour, xanthan gum, cream of tartar, sugar and yeast, and whisk with a separate handheld whisk to combine well. Add the kosher salt, and whisk once again to combine.

Step 5

Add the vinegar, egg, milk, butter, and water roux, and mix well on low speed. Once the dry ingredients have been incorporated into the wet ingredients, turn the mixer up to high speed and allow to mix for about 3 minutes. The dough should be shaggy and tacky to the touch.

Step 6

Turn the dough out onto a very lightly floured surface, and divide into 3 equal portions (each about 350 grams). Work with one piece of dough at a time.

Step 7

Pat out the first piece of dough into a flat disk about 3/4-inch thick. Fold the disk loosely in half from one short end to the other. Repeat with the remaining pieces of dough.

Step 8

Stack the folded pieces of dough one behind the other, the rounded (folded) part facing up. The shaped pieces of dough should fit snugly in the pan from end to end, and will rise up, not out.

Step 9

Spray the dough lightly with warm water, and cover the pan loosely with plastic wrap. Place in a warm, draft-free location to rise until it has reached about 150% of its original volume.

Step 10

While the dough is rising, preheat your oven to 350°F.

Step 11

Once the dough has finished rising, remove the plastic wrap, brush lightly with the optional melted butter, and place in the center of the preheated oven. Bake for 45 minutes.

Step 12

Remove the bread from the loaf pan and transfer to a rimmed baking sheet. Return the bread on the baking sheet to the oven.

Step 13

Continue to bake until the loaf sounds hollow when thumped on the side and reads about 190°F when an instant-read thermometer is inserted in the center of the loaf and is lightly golden brown all over (about another 10 minutes). Remove the loaf from the oven and transfer it to a wire rack to cool completely before slicing and serving.