Preheat the oven to 350°F. Line a 9 x 13 pan with a foil sling and lightly grease with cooking spray.
Place the halva, miso, and brown sugar in a large bowl and set aside.
Place the butter in a small saucepan and cook over medium heat, until it smells nutty and fragrant and is a light brown color.
Pour the hot, browned butter into the bowl with the miso, halva, and brown sugar. Whisk everything together until a smooth-ish mixture forms. It won’t come together completely, but that’s okay. Let the mixture cool for about 10 minutes before moving on to the next step (you don’t want to accidentally cook the eggs!).
Add the eggs in one at a time, whisking vigorously after each addition. You’re looking to get a nice, smooth and shiny batter. Then whisk in the vanilla, kosher salt, and baking powder. Add in the flour and stir until it is only partially combined. Then stir in the chopped chocolate and mix until everything is just combined.
Pour the batter into your prepared pan and spread it into an even layer. Bake for 16 – 19 minutes or until the edges are set, but the middle still looks slightly underdone. It’s super important not to over bake these, so if anything err on the side of under baking. Let cool in the pan for at least one hour before cutting into squares.