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.