Position a rack in the center of the oven and preheat to 350º. Put sling of parchment paper or heavy aluminum foil in an 8x8" square baking pan, or grease the pan.
Spread the pecans on a small sheet pan and roast in the oven until golden and fragrant, 10 - 12 minutes. Remove and let cool, then break or chop into coarse pieces.
Meanwhile, spread the coconut on another small baking pan and bake for 5 minutes until golden, stirring once or twice. Set aside to cool.
In a large bowl, whisk together the sugar and egg. Slowly pour in the melted butter, whisking to combine, and add the rum and vanilla.
In a small bowl, whisk or sift together the flours, xanthan gum, baking powder and salt. Stir the dries into the wets. Stir in the cooled pecans, coconut, and chocolate chunks.
Spread the dough evenly into the parchmented pan. (It will look like not enough, but don't worry, it will rise in the oven.)
Bake until puffed, shiny and slightly cracked on top, and a toothpick inserted comes out with a few large, wet crumbs clinging (don't overbake), 15 - 20 minutes. Let cool at least 20 minutes before cutting (longer for cleaner-looking bars). Trim away the outer 1/2", then cut into 16 bars.
The bars are best the day they are made, but will keep, in an airtight container, for several days.
Adapted from Cook's Illustrated.If gluten isn't an issue for you, you can substitute 3/4 cup all purpose, whole wheat, or whole or white spelt flour for the rice and tapioca flours and omit the xanthan gum.Sticky rice flour, sometimes called "sweet" or "glutinous," is available at most well-stocked grocers. I usually use Mochiko brand. Regular rice flour may make for crumbly congo bars that don't hold together. If that's all you've got, try doubling the xanthan gum.Nutritional values are based on one of sixteen bars.