Preheat the Oven: Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan or line it with parchment paper for easy removal.
Mix the Wet Ingredients: In a large mixing bowl, combine the melted butter, buttermilk, eggs, vanilla extract, and sugar. Whisk until smooth and well combined.
Combine the Dry Ingredients: In another bowl, sift together the flour, cocoa powder, baking powder, and salt. This step helps prevent clumps in the dry ingredients.
Incorporate the Dry Ingredients into the Wet Ingredients: Gradually fold the sifted dry ingredients into the wet mixture. Stir gently until the batter is smooth and evenly combined. Avoid overmixing to keep the brownies tender.
Add Optional Add-ins: If desired, fold in chocolate chips, chopped nuts, or a pinch of cinnamon or coffee powder for added flavor and texture.
Bake the Brownies: Pour the batter into the prepared pan, smoothing it into an even layer. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (not wet batter).
Cool and Cut: Allow the brownies to cool in the pan on a wire rack for about 30 minutes before cutting them into squares or rectangles.