Melt butter and chocolate either in the microwave or in a bowl over a pot of simmering water. If you melt it in the microwave, heat on high for 30 seconds at a time, stirring in between until melted. Do not overheat. Transfer to a large bowl and cool.
In a medium bowl, combine flour blend, baking powder, salt, and xanthan gum
In the bowl with the chocolate, add 1 cup of sugar, eggs, and vanilla and mix well. Stir in flour mix until blended. Cover and chill at least 1 1/2 hours or until dough is firm enough to shape.
Preheat oven to 300F. Prepare 2-3 cookies sheets with parchment paper or silicone baking mats.
Portion pieces of the dough which are slightly larger than a tablespoon. Roll into a ball and then roll in the remaining sugar. The dough gets sticky.
Place 2” apart on the sheets. Bake 18-20 minutes, rotating pans in the oven halfway through. Bake until crackled on top and firm to touch.
Place cookie sheet on wire rack and cool at least 10 minutes before removing. Using a spatula, transfer cookies to the rack. Cool cookies completely.