Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan.

In a medium saucepan, combine the semisweet chocolate and unsalted butter over low heat until melted and smooth.

In a large bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking powder, baking soda, ground cinnamon, ground ginger, ground nutmeg, and ground cloves.

In the bowl with the melted mixture, add light brown sugar, molasses, eggs, and vanilla extract and whisk until glossy and combined.

Gradually fold the dry ingredients into the wet mixture until just combined, being careful not to overmix.

Fold in the chopped crystallized ginger.

Pour the batter into the prepared pan and bake for 25-30 minutes until a toothpick comes out with a few moist crumbs.

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

In a mixing bowl, beat together cream cheese and softened butter until fluffy, then add powdered sugar, ground nutmeg, and eggnog until combined.

Spread the Eggnog Cream Cheese Frosting over the cooled brownies and dust with additional ground nutmeg or cinnamon if desired.

Cut into squares and serve, allowing the flavors to deepen overnight if possible.