Preheat your oven to 400°F. Grease well a 6-cup popover pan (or a regular muffin tin with very deep wells) with unsalted butter and set it aside.
In a large bowl, whisk together the flour, xanthan gum and salt. Add the butter, eggs, and milk, whisking well after each addition until the batter is smooth.
Place the greased popover pan in the hot oven for 2 minutes.
Remove from the oven and immediately fill the wells of the pan 3/4 of the way full.
Return the pan to the oven and bake for 20 minutes.
Turn the oven down to 325°F, and continue baking until lightly golden brown on top (about another 10 minutes).
Serve immediately, plain, with butter, or with your favorite jam or preserves.