In small bowl, mix flour blend, xanthan gum, cinnamon, baking powder and salt. In large bowl, beat 1/3 cup butter and 1/2 cup brown sugar with electric mixer on medium speed until light and fluffy. Beat in eggs 1 at a time, until just combined. Stir in vanilla. On low speed, alternately stir in flour blend mixture and buttermilk; mix until just combined. Pour batter evenly into muffin cups (cups will be full). Place muffin pan on foil-lined cookie sheet.