Preheat the oven to 350 (F), and prepare your baking pan by spraying with cooking spray. Line with parchment paper and grease paper. Set pan aside.
In a double boiler (or, a homemade version with a heatproof bowl sitting on top of a saucepan), combine 1 cup (2 sticks) unsalted butter with 8 ounces Pitch Dark chocolate. Use a rubber spatula to stir constantly until melted and smooth, about 5 minutes. Remove from heat and set aside.
In a medium bowl, whisk together 4 eggs in a large bowl. Add 1 cup granulated sugar, 1 cup dark brown sugar, 1 tablespoon pure vanilla extract, and 1/2 teaspoon kosher sea salt. Whisk until fully incorporated β the mixture should be a golden tan color. Whisk in the chocolate mixture (from the second step).
Once the mixture is a uniform dark brown, use a rubber spatula to fold in 1/2 cup all-purpose flour and 1/2 cup graham flour. Be careful not to overmix during this part! Simply fold in until the flour is incorporated βit's okay to have one or two small flour streaks left in the batter. Scatter 1 cup whole and halved walnuts over the batter and stir once or twice until fully incorporated.
Pour batter into prepared pan, using the rubber spatula to spread evenly. Bake for 30 - 35 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool completely on a wire rack, preferably overnight, before cutting and serving.
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