Preheat oven to 350 °F. Grease 8x8x2-inch pan with non-stick cooking spray. Set aside. In small bowl, whisk together brown rice flour, almond flour, cornmeal, sweet rice flour, baking powder, and xanthan gum. In bowl of stand mixer fitted with paddle attachment, combine butter and granulated sugar. Cream, on medium-high speed, until butter is light and almost white, about two minutes. Add eggs, one at time, scraping down the bowl between each addition. Allow butter-sugar mixture to fully incorporate each egg before adding the next.