01 - Preheat the oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.
02 - Melt the butter in a small saucepan over medium heat. Continue cooking, swirling occasionally, until the butter foams and turns a deep golden brown with a nutty aroma, 4–6 minutes. Immediately pour into a large mixing bowl to cool slightly.
03 - Add the chopped chocolate to the brown butter and stir until completely melted and smooth.
04 - Whisk in the granulated sugar and brown sugar until well combined and the mixture is glossy.
05 - Add the eggs one at a time, whisking thoroughly after each addition. Stir in the vanilla extract.
06 - In a separate bowl, whisk together the sweet rice flour, cocoa powder, baking powder, and salt until evenly distributed.
07 - Add half of the dry ingredients to the wet mixture, stirring until just combined.
08 - Pour in the milk and heavy cream; mix until smooth.
09 - Add the remaining dry ingredients and fold gently until no streaks remain. Do not overmix.
10 - Pour the batter into the prepared pan and smooth the top with a spatula.
11 - Bake for 40–45 minutes, or until the top is set and a toothpick inserted in the center comes out with a few moist crumbs.
12 - Cool completely in the pan before lifting out and slicing into 16 squares.