Preheat your oven to 350°F (175°C), then grease a 9x13 inch baking pan with butter or cooking spray, ensuring the bottom and sides are well-coated.
In a large mixing bowl, combine the melted butter, granulated sugar, and packed brown sugar. Stir these ingredients together until the mixture is thoroughly combined and smooth.
Add the eggs to the bowl one at a time, stirring well after each addition until the mixture is creamy. Then, blend in the vanilla extract, ensuring an even distribution.
In a separate bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking powder, and salt until they are uniformly mixed.
Gradually add the dry ingredients into the wet mixture, stirring just until combined. Be careful not to overmix; a few lumps are perfectly fine. Once combined, pour the thick batter into the greased baking pan and spread it out evenly.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out mostly clean with a few moist crumbs attached. Allow the brownies to cool completely in the pan set on a wire rack.
For the eggnog frosting, start by beating the softened butter with an electric mixer on medium speed until it is creamy and light in texture. Gradually incorporate the powdered sugar while mixing on low speed until everything is well combined and smooth.
Mix in the eggnog, ground nutmeg, and cinnamon, blending until all ingredients are fully incorporated. Adjust the thickness of the frosting by adding either more powdered sugar for a thicker consistency or a splash of eggnog for a lighter spread.
Once the brownies have cooled fully, spread the luscious eggnog frosting evenly over the top using a spatula or a butter knife, creating a smooth layer.
For an optional festive touch, pipe whipped cream along the edges and sprinkle a delicate layer of nutmeg on top. Cut the frosted brownies into squares and serve wonderfully!
Notes
For a festive touch, add whipped cream and nutmeg on top.