How to Make It
Combine buttermilk and 1 tablespoon granulated sugar in a small saucepan, stirring with a whisk. Cook over medium heat until thermometer registers 110°. Remove from heat. Dissolve yeast in buttermilk mixture; let stand 5 minutes.
Weigh or lightly spoon flours into dry measuring cups; level with a knife. Combine flours, baking powder, xanthan gum, 1/4 teaspoon cinnamon, baking soda, and salt in a large bowl; beat with a mixer at medium speed until blended. Add buttermilk mixture, oil, vanilla, and eggs, beating until a soft, sticky dough forms.
Sprinkle pecans in bottom of a 10-inch Bundt pan coated with cooking spray. Combine 1/4 cup granulated sugar and 1/2 teaspoon cinnamon in a small bowl.
Scoop dough using a 1 1/2-inch ice-cream scoop coated with cooking spray; roll dough ball in cinnamon-sugar mixture, and place in Bundt pan. Repeat procedure with remaining dough and cinnamon-sugar mixture. Cover pan loosely with plastic wrap, and let rise in a warm place (85°), free from drafts, 1 1/2 hours.
Preheat oven to 350°.
Combine brown sugar and melted butter, stirring until smooth. Pour evenly over dough balls in pan. Bake at 350° for 35 minutes or until top of bread is golden brown. Place a serving plate on top of pan; invert bread onto plate. Cool 15 minutes before serving.