Preheat oven to 350 degrees F. Lightly oil the bottom of a 9" square pan and line with parchment paper. Fold parchment so edges are crisp and leave folded edges over the edge of pan to serve as handles to lift bars from pan after baking. Oil parchment.
In the bowl of your mixer, whisk together brown rice flour, potato starch, tapioca starch, sugar, first measure of brown sugar, and the xanthan or guar gum until well blended. Add first measure of cold margarine or coconut oil and whisk with mixer until dough is crumbly but holds together when squeezed in hand. Place dough into prepared pan and press into an even layer. Bake at 350 degrees F for 10 minutes.
While crust is baking, whisk second measure of margarine or coconut oil with remaining brown sugar, and honey or maple in a sauce pan over medium heat until sugar has just dissolved. Remove from heat.
In mixing bowl, whisk egg. While mixer is on low, slowly drizzle warm sugar into whisking egg. Add coconut milk, vanilla and pecans. Pour mixture into pan over the partially baked crust.
Bake pecan bars for 25 minutes until center is set. Cool completely, then chill before slicing. Use parchment paper to lift the chilled bars from pan when ready to slice.
Serve at room temperature or slightly chilled.
To freeze: Layer in a freezer safe container separated by waxed paper. Remove as much air as possible, seal, label and freeze.
To serve: Thaw. Serve at room temperature or slightly chilled.