Preheat your oven to 350°F (175°C) and line a 13x9-inch baking pan with parchment paper. If you don't have parchment paper, grease the pan with butter or cooking spray.

Melt the butter and chocolate: In a heat-resistant bowl, mix together the butter and chopped chocolate. Microwave in 10-second intervals, stirring in between, or use a double boiler (place a heatproof dish over simmering water). Stir the mixture until it becomes smooth, then set it aside to cool for a few minutes.

Mix the eggs and sugar: In another bowl, beat the eggs with an electric mixer for about 30 seconds. Add the sugar and vanilla extract, and continue mixing for 3-5 minutes, until the mixture becomes pale and slightly thickened.

Combine the chocolate mixture: Stir the cooled melted butter and chocolate into the egg mixture.

Sift the dry ingredients: Sift the flour, cocoa powder, and salt over the egg mixture. Gently stir everything together until well combined. The batter should have a smooth, thick texture.

Bake: Transfer the batter to the prepared pan and smooth it out evenly. Bake for 18-24 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Allow the brownies to cool on a rack for at least two hours.

As the brownies cool, start preparing the icing:

Beat the butter and cream cheese: In a large mixing bowl, beat the softened butter and cream cheese together until they are smooth and creamy. Make sure the butter and cream cheese are at room temperature for the best consistency.

Add the powdered sugar: Gradually mix in the sifted powdered sugar, a little at a time, until the icing is smooth and spreadable. Add the vanilla extract and salt, and continue beating until everything is well combined.

Frost the brownies: Once the brownies have completely cooled, spread the icing evenly on top using a spatula to create a smooth finish.