How to Make It
Preheat oven to 375°.
Line a 9-inch square metal baking pan with parchment paper.
To prepare crust, weigh or lightly spoon white rice flour, cornstarch, and sorghum flour into dry measuring cups; level with a knife. Place white rice flour, cornstarch, sorghum flour, 1/2 cup brown sugar, and next 4 ingredients (through salt) in a food processor; pulse until mixture resembles fine meal. Press flour mixture into bottom of prepared pan. Bake at 375° for 15 minutes or until edges of crust are slightly browned.
While crust bakes, combine syrup and next 3 ingredients (through 1 tablespoon butter) in a medium saucepan; bring to a boil over medium heat. Cook 2 minutes, stirring constantly with a whisk until sugar dissolves. Remove from heat; add pecans and 1/2 teaspoon vanilla, stirring with a whisk. Pour mixture into hot crust. Bake at 375° for 10 minutes or until filling is bubbly. Cool completely in pan. Cut into 20 bars.