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