Preheat oven to 350F. Line 24 muffin cups with paper liners. Set aside.
In a large bowl of a stand mixer fitted with the paddle attachment (you can also use a hand mixer), combine gluten-free flour blend, xanthan gum, cocoa powder, baking powder, baking soda, salt and granulated sugar and which to combine.
Add vegetable oil, vanilla extract, eggs and buttermilk and mix for 1-2 minutes.
Pour in the hot water and mix for an additional 1-2 minutes. The batter will be very thin. Let sit for 2-3 minutes to allow the batter to slightly thicken.
Pour into cupcakes liners and fill about 1/2 full, filling all 24 cups.
Bake at 350F for 23-25 minutes, until the tops of the cupcakes spring back and when a toothpick is inserted it comes out clean.
Let cool completely before topping with buttercream.