Preheat oven to 350°F (175°C). Grease a 9x5-inch loaf pan and line with parchment paper, leaving an overhang for easy removal.
In a large mixing bowl, whisk together the melted butter, sugar, eggs, and vanilla extract until smooth and glossy.
Sift in the flour, cocoa powder, salt, and baking powder. Stir until just combined. Fold in chocolate chips.
Pour half the batter into the loaf pan, then drizzle half the hot fudge sauce over it. Swirl gently with a knife. Repeat with the remaining batter and fudge. Bake for 45–50 minutes, or until a toothpick comes out with moist crumbs (avoid overbaking for a fudgy texture). Let cool before slicing.