Preheat oven to 350°F. In the bowl of a food processor, combine brown rice flour, cornstarch, dark brown sugar, baking powder, and salt. Pulse to combine. (If you don't have a food processor, whisk dry ingredients together in a medium mixing bowl.)
Turn dough out onto a lightly rice floured piece of 12x16 parchment paper. Pat dough into a rectangle. Dust the top of the dough lightly with rice flour.
Carefully remove top piece of parchment paper. Transfer dough, with bottom parchment paper, to a 12 x 18 baking pan.
Chill dough for 10 minutes. Bake for 15 minutes or until evenly brown.
Remove graham crackers from the pan and place on a wire rack to cool. (I slide the parchment, with the crackers, directly onto the cooling rack.) Allow crackers to cool completely. Break along scored lines.