In a large bowl, whisk together the gluten-free flour, xanthan gum, salt, granulated sugar, and gluten-free baking powder.
In a separate medium bowl, whisk together the milk, egg, and pure vanilla extract.
Pour the wet ingredients into the dry ingredients and mix until just combined. Be careful not to overmix.
Stir in the ¼ cup melted butter until fully incorporated.
Gently fold in the ¼ cup softened butter, creating small pockets and swirls for the 'cinnamon roll' effect. Do not fully combine.
Divide the batter evenly into a greased muffin tin, filling each cup about two-thirds full.
Bake according to package directions for muffins, or until a toothpick inserted into the center comes out clean.