In a small microwavable dish, melt 1/4 cup butter and cool.
Line a large cookie sheet with parchment paper and place 8 silicone rings (4-inch round, 3/4-inch high) on top, rim side up.
Lightly butter the insides and rims of the rings with some of the melted butter. Reserve the rest of this butter for basting the tops of the buns.
In a large mixing bowl, whisk together flour, sugar, yeast, xanthan gum and salt until well combined.
Heat the milk to 100 degrees. Do not over-heat. If you do, let it cool to 100 degrees before proceeding as the higher heat can kill off the yeast.
Add the warmed milk, sour cream, 1/4 cup melted, cooled butter, vinegar and egg to the dry ingredients and mix at low speed until just combined. Scrape down the sides of the bowl as necessary. Using a medium-high speed, continue to mix for about 3 minutes.
Divide the batter equally between the 8 prepared rings, approximately one slightly rounded 1/3 cup each.
Using the reserved melted butter, baste the tops of the buns. It may seem like a lot of butter, but use it all. It helps to add flavor.
Allow the buns to rise in a warm, draft-free location for approximately 30 to 35 minutes.
Preheat the oven to 375 degrees.
Place the buns in the middle of the oven and bake until the internal temperature of the buns reads at least 210 degrees but aiming for 215 degrees, approximately 25 minutes. Do not under cook!
Once done, allow the buns to rest on the cookie sheet for 10 minutes, then remove to a wire rack and cool completely.
Do not attempt to slice the buns until they are completely cooled. Cutting into them while they are still warm will result in a gummy texture.
Do not refrigerate these buns. For best results store tightly wrapped at room temperature and use within 2 days or freeze until ready to use.
Slice before freezing for easy and quick defrosting. When using the buns after freezing, a few seconds in the toaster will help to bring back that "just baked" texture.
These work extremely well not only for hamburger buns but for sandwich rolls as well. Makes 8 buns.