Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan or line it with parchment paper.
In a microwave-safe bowl or using a double boiler, melt the butter until it's completely liquid. Let it cool slightly.
In a mixing bowl, whisk together the granulated sugar and eggs until well combined.
Stir in the vanilla extract into the sugar and egg mixture.
In a separate bowl, sift together the flour, cocoa powder, salt, and baking powder.
Gradually add the dry ingredients to the wet ingredients, mixing until a smooth batter forms.