Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan with butter or spray, or line it with parchment paper, leaving some overhang for easy removal later.
In a large mixing bowl, combine the melted butter, packed brown sugar, and granulated sugar. Whisk them together until well mixed and creamy.
Add the eggs to the mixture one at a time, ensuring to mix thoroughly after each addition. Stir in the vanilla extract until the batter is smooth and cohesive.
In a separate bowl, sift together the all-purpose flour, cocoa powder, ground ginger, ground cinnamon, ground nutmeg, baking powder, and salt to ensure even distribution of the dry ingredients.
Gradually incorporate the dry ingredients into the wet mixture, folding gently with a spatula until just combined. Be careful not to overmix; the batter should remain thick and slightly lumpy.
Carefully fold in the unsweetened applesauce and molasses, which will imbue the brownies with a rich gingerbread flavor. Mix until everything is delectably smooth and well combined.
Pour the brownie batter into your prepared baking pan, using a spatula to spread it evenly across the base.
Place the pan in the preheated oven and bake for 25-30 minutes. The brownies are ready when a toothpick inserted into the center comes out with a few moist crumbs clinging to it (avoid any wet batter).
Once baked, allow the brownies to cool in the pan for about 10 minutes. Then, using the edges of the parchment paper, carefully lift them out and transfer to a wire rack to cool completely.
After they have cooled, cut the brownies into squares to serve. If desired, dust the tops with powdered sugar for an extra festive touch.
Notes
Serve warm with vanilla ice cream and caramel sauce for an indulgent treat.