Line a muffin pan with paper liners and set aside.

In your mixing bowl, combine gluten-free flour, xanthan gum, sugar, baking soda, and salt and set aside.

In a medium saucepan, combine butter, cocoa, and 1 cup water. Bring just to boiling, whisking constantly. Remove from heat and add to dry ingredients, beat until combined.

Add buttermilk, eggs, and vanilla and beat for one minute on low-medium speed.

Pour batter into cupcake liners. I use a large scoop that measures about 1/3 cup batter into each opening.

Bake at 350° for 17 minutes or until a toothpick comes out clean or with a few crumbs (no wet batter).

Let cool in pan for 4 minutes, then remove cupcakes. Wait to frost until completely cool.

Using a medium glass bowl, melt the stick of butter in microwave. Whisk in cocoa powder, then heat for 1 minute. Add powdered sugar and milk alternately, beating with a hand mixer until creamy. Add vanilla last, beating in until mixed well.

If you use this frosting while still warm, it is easily spreadable and has the glossy finish when dried like you see in the photo but can slide off of whatever you are frosting. If you let it cool completely, then it firms up and can be placed into a pastry bag for piping for decorating cakes or cupcakes. The icing can be refrigerated to use later; just microwave 10 seconds at a time, and stir, until you get to the right consistency for piping or spreading.