1Combine ice water, sour cream, and vinegar together in a bowl. Process flour blend, sugar, salt and xanthan gum together in food processor until combined. Scatter butter over the top and pulse mixture until butter is pea sized.
2Pour half of sour cream mixture over flour mixture and pulse until incorporated. Pour remaining sour cream mixture over flour mixture and pulse until dough comes together.
3Divide dough into 2 even pieces. Flatten each piece into a 5" disc and tightly wrap in plastic wrap and refrigerate at least one hour. Before rolling let dough sit out for 15 minutes to soften slightly. Dough can remain refrigerated for up to two days.