Preheat your oven to 350°F (175°C).
Line a 9×13-inch baking pan with parchment paper, leaving a bit of overhang on the sides for easy removal. Lightly grease the parchment.
Melt the butter in a medium saucepan over medium heat. Once melted, remove from the heat and stir in the sugar until fully dissolved and glossy.
Add the eggs one at a time, whisking well after each addition. Stir in the vanilla extract.
In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt.
Slowly add the dry ingredients to the wet mixture, stirring until the batter is fully combined and smooth.
Pour the batter into the prepared pan and spread evenly with a spatula.
Bake for 25–30 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs.
Cool completely in the pan on a wire rack before frosting.