Preheat your oven to 350°F (175°C) and prepare a 9x13-inch baking pan by greasing or lining it.

Melt the semisweet chocolate and unsalted butter in a medium saucepan over low heat, stirring until smooth. Let it cool slightly.

Whisk together all-purpose flour, cocoa powder, baking powder, baking soda, ground cinnamon, ground ginger, ground nutmeg, and ground cloves in a separate bowl.

Combine the cooled chocolate mixture with light brown sugar, eggs, and vanilla extract in a large mixing bowl. Whisk until glossy.

Fold the dry ingredients into the wet ingredients until just combined. Do not overmix.

Gently fold in the chopped crystallized ginger.

Pour the batter into the prepared pan and bake for 25-30 minutes. Check with a toothpick for moist crumbs.

Let the brownies cool in the pan for 15 minutes, then transfer to a wire rack to cool completely.

Beat together cream cheese and unsalted butter, then gradually add powdered sugar, ground nutmeg, and eggnog until smooth.

Spread the frosting over the cooled brownies, adding extra spices on top if desired.

Cut into squares and let them sit overnight for best flavor before serving.