Pre-heat the oven to 355 ºF (180 ºC) and line two 7 inch round cake pans with greaseproof/baking paper.
In a large bowl, sift together the gluten free flour, xanthan gum, baking powder, baking soda and cocoa powder.
Add the granulated sugar and salt, and whisk well.
Add the eggs, milk and vegetable/sunflower oil. Whisk well, until you get a smooth cake batter.
Add the boiling hot water, and whisk until you get a runny cake batter with no lumps.
Evenly distribute the cake batter between the two lined cake pans, and bake in the pre-heated oven at 355 ºF (180 ºC) for about 40 minutes or until spongy to the touch and an inserted toothpick comes out clean.
Allow to cool.
In a stand mixer, or using a hand mixer with the double beater attachments, beat the butter for 2 - 3 minutes, until pale and fluffy.
Add the powdered sugar, and beat for a further 5 minutes.
Add the cocoa powder and salt, and beat until evenly distributed in the buttercream.
Add in the melted (and cooled) dark chocolate, and beat until you get a rich, fluffy chocolate frosting with an even chocolate brown colour.
If the sponges are domed, level them out by cutting off the domed tops with a serrated knife.
Place the bottom sponge layer on a cake stand, and spread a generous layer of frosting on top, but leave enough for frosting the outside of the cake. Then, place the other sponge on top of the frosting.
Use the remaining chocolate buttercream to frost the outside of the cake. Decorate the cake by creating swirls of buttercream with an offset spatula or the back of a spoon.
The gluten free chocolate cake keeps well in the fridge, in a closed container or wrapped in cling film, for 3 - 4 days.