01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving overhang on two sides for easy removal.
02 - Melt butter in small saucepan over medium heat. Continue cooking, swirling occasionally, until butter foams and turns deep golden brown with nutty aroma, 4-6 minutes. Pour into large mixing bowl to cool slightly.
03 - Add chopped chocolate to brown butter, stirring until melted and smooth.
04 - Whisk in granulated sugar and brown sugar until well combined.
05 - Add eggs one at a time, whisking thoroughly after each addition. Stir in vanilla extract.
06 - In separate bowl, whisk together sweet rice flour, cocoa powder, baking powder, and salt.
07 - Add half of dry ingredients to wet mixture, stirring until just combined. Pour in milk and heavy cream, mixing until smooth. Add remaining dry ingredients and fold gently until no streaks remain.
08 - Pour batter into prepared pan, smoothing top with spatula. Bake for 40-45 minutes until top is set and toothpick inserted in center comes out with a few moist crumbs.
09 - Cool completely in pan before lifting out and slicing into squares.