While heating milk/cream mixture, whisk egg yolks in medium bowl. Gradually add 2/3 cup of sugar and xanthan gum and whisk until mixture is thick and pale yellow. Temper egg yolks by slowly pouring 1 cup hot milk mixture at a time, whisking constantly. Return egg yolk mixture to to saucepan and place over low heat. Cook, stirring frequently with a wooden spoon until the custard reaches a temperature of 185°F. Pour the mixture through a fine-mesh strainer into a clean bowl and cool to room temperature, stirring every 5 minutes. Once room temperature, cover and place in refrigerator for at least 4 hours or overnight.