Preheat oven to 350°F. Grease a 9x5 loaf pan and line with parchment paper.
Whisk melted butter and sugar together. Add eggs one at a time, then stir in vanilla.
Fold in cocoa powder, flour, baking powder, and salt until just combined.
Stir in chocolate chips and hot fudge sauce. Do not overmix once fudge is added.
Pour batter into the pan. Spoon extra fudge on top and swirl with a toothpick.
Bake for 45 to 50 minutes until the top is set and a toothpick comes out with moist crumbs.
Cool in the pan for 15 minutes, then lift out using the parchment and slice once cooled.