1¾ cups (350 grams) granulated sugar1¼ cups (284 grams) unsalted butter, melted⅔ cup (147 grams) firmly packed light brown sugar1 teaspoon (4 grams) vanilla extract5 large eggs (250 grams), room temperature1 cup (85 grams) unsweetened natural cocoa powder, sifted¾ cup (94 grams) all-purpose flour3¾ teaspoons (7.5 grams) Mulled Spice Mix (recipe follows)1 teaspoon (3 grams) kosher salt¾ teaspoon (3.75 grams) baking powder1½ teaspoons (5 grams) finely chopped port-soaked orange peel (reserved from Spiced Port Syrup )Ganache Frosting (recipe follows)Spiced Port Syrup (recipe follows)Garnish: Mulled Spice Mix (recipe follows)
Preheat oven to 350°F (180°C). Spray a 9-inch square baking pan with cooking spray; line pan with parchment paper, letting excess extend over sides of pan.
In a large bowl, whisk together granulated sugar, melted butter, brown sugar, and vanilla until well combined. Add eggs, one at a time, beating until well combined after each addition.
In a large bowl, whisk together cocoa, flour, Mulled Spice Mix, salt, and baking powder. Gradually stir cocoa mixture into sugar mixture until combined. Fold in chopped orange peel, breaking up any clumps if needed. Spoon batter into prepared pan, smoothing top.
Bake until a wooden pick inserted in center comes out with just a few moist crumbs, 45 to 50 minutes. Let cool completely in pan on a wire rack.
Spread Ganache Frosting onto cooled brownies, using a small offset spatula or the back of a spoon to create swirls. (For deeper swirls, refrigerate brownies for about 15 minutes to slightly firm up frosting before swirling.) Using excess parchment as handles, remove from pan. Cut into bars. Drizzle with Spiced Port Syrup just before serving. Garnish with Mulled Spice Mix, if desired.