Preheat the oven to 350°F (175°C) and grease a 9×5-inch loaf pan, dusting lightly with flour. Let the butter and eggs sit at room temperature for about 30 minutes.

Whisk together the flour, cocoa powder, baking powder, and salt in a medium bowl and set aside.

Cream the butter and sugar in a large bowl until light and fluffy, about 3-4 minutes.

Add the eggs one at a time, beating well after each addition, then stir in the vanilla extract.

Gradually add the dry ingredients to the wet ingredients, alternating with the milk, starting and ending with the dry ingredients. Mix just until combined, then fold in the melted chocolate if using.

Pour the batter into the prepared pan and bake for 60-70 minutes, or until a toothpick inserted into the center comes out clean.

Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.