Preheat your oven to 350°F (175°C). Grease a 9x5-inch loaf pan with butter and dust with flour, or line it with parchment paper.

In a medium bowl, whisk together flour, cocoa powder, baking powder, and salt. Set aside.

In a large mixing bowl, combine melted butter, granulated sugar, and brown sugar. Beat until smooth. Add eggs and vanilla extract, and mix until fully incorporated.

Gradually add the dry ingredients to the wet mixture in three parts, alternating with milk or buttermilk. Stir gently after each addition. Fold in chocolate chips, if using.

Pour the batter into the prepared loaf pan and spread evenly. Bake for 50 to 60 minutes, until a toothpick inserted in the center comes out with a few moist crumbs rather than wet batter.

Let the loaf cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.