Preheat oven to 350°C/175°C. Grease and line the bottoms of two 8 inch round cake pans. Set aside.
In a large mixing bowl: sift together the flour, xanthan gum, baking powder and salt. Sift until well combined. Set aside.
In another large mixing bowl beat the eggs with an electric mixer on high, about 20 seconds. Gradually add the sugar as the mixer is running. Continue to beat the eggs and sugar together on high, about 3-5 minutes until the mixture is pale-yellow, thick, glossy and has tripled in volume.
In a small sauce pot, warm the butter and milk over a low heat. Warm until the milk is just about to boil and the butter has melted. Add the vanilla extract.
In three stages, add the flour mixture to the egg mixture. Use a wooden spoon or rubber spatula to fold the flour into the eggs. Work slowly and carefully to avoid over mixing the batter. Continue until all the flour has been added and fully mixed in to the batter. The batter should be thick and airy.
Slowly pour the melted butter, milk and vanilla into the batter. Mix gently with a large whisk until just combined.
Pour the batter into the two prepared cake tins. Gently lift up the tins and drop them on a counter/table surface to get rid of air bubbles.
Place in the middle of the oven and bake for 28-30 minutes. To check for doneness, insert a knife or tooth pick into the centre of the cake. If it comes out clean it's done.
Let cool in the tin for about 10 minutes then flip over onto a cooling rack. Let cool completely before frosting or filling.