Step 1Heat the oven to 350 degrees. Lightly coat a 9-by-13-inch metal baking pan with nonstick cooking spray, line the bottom and sides with parchment paper and spray the parchment.
Step 2Whisk the rice flour, baking powder and turmeric in a large bowl. Beat the eggs and sugar in another large bowl with an electric mixer on medium-high speed until pale yellow. Reduce the speed to medium and pour in the coconut milk in a steady stream, then the butter and vanilla. Scrape down the bowl. Reduce the speed to low and gradually add the dry ingredients, beating until smooth. Alternatively, whisk the ingredients by hand, adding them in the order above. Pour the batter into the prepared pan.
Step 3Bake until golden brown and the edges are just starting to pull away from the sides of the pan, about 1 ½ hours.
Step 4Cool completely in the pan on a wire rack. Cut into quarters lengthwise, then cut each quarter into 1-inch-wide bars to form 48 rectangles.