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["Preheat oven to 350u00b0F. Butter and flour an 8x8"" baking dish, shaking out excess flour. Toast walnuts, if using, on a rimmed baking sheet, tossing occasionally, until golden brown, 8-10 minutes; let cool. Increase oven temperature to 375u00b0F.", "Whisk cocoa, salt, baking powder, and 1 cup flour in a medium bowl to combine and work out any lumps. Heat chocolate, sugar, and 1/2 cup butter in a large heatproof bowl set over a saucepan of simmering water (bowl should not touch water), stirring constantly, until smooth. Remove from heat and add eggs one at a time, beating vigorously with a wooden spoon to blend after each addition. Mix in vanilla, then add dry ingredients and mix until just combined. Fold in reserved walnuts (do not overmix).", "Scrape batter into prepared pan; smooth top. Bake until a tester inserted into the center comes out clean, 30-40 minutes. Let cool.", "Heat chocolate, butter, rum, cocoa powder, vanilla, and salt in a heatproof bowl set over a saucepan of simmering water (bowl should not touch water), stirring constantly, until smooth.", "Pour glaze over brownies and top with walnuts, if using. Chill until glaze is set, at least 1 hour. Slice into 12 brownies.", "Brownies can be made 1 day ahead. Keep chilled."]
