In a large bowl, whisk together flour, starch, xanthan gum, baking powder, baking soda, salt, and sugar.
In another bowl or measuring cup, mix together buttermilk, eggs, and melted butter.
Create a well in the middle of the flour mix, and then begin to pour the buttermilk mixture into the well, while whisking quickly to combine the mixtures. Continue whisking until smooth and the lumps are gone. The mixture will be thick.
Heat a greased griddle, skillet, or pan on medium heat. Pour a little less than 1/4 cup of batter onto the heated surface (do not pour in a swirling fashion, but straight down). Cook until the edges just start to set, and then take a little bit more batter (maybe about a tablespoon or so) and pour into the middle of the partially set pancake. This will create extra thick pancakes.
Cook until bubbles start to break through the batter, the edges are set, and the the top around the edges lose their shine (approximately 2 minutes). Flip pancake and cook until set (approximately another two minutes). Cook as many pancakes as you can on the surface, and repeat steps until all the remaining batter is gone.