Preheat the oven to 350°F (175°C). Grease a loaf pan with butter or non-stick spray.

In a mixing bowl, whisk together granulated sugar, brown sugar, and melted butter until smooth.

Add eggs and vanilla extract, whisking until fully combined.

In another bowl, mix flour, cocoa powder, salt, and baking powder.

Gradually add the dry ingredients to the wet ingredients, stirring until just combined.

Pour half of the brownie batter into the prepared loaf pan.

Drizzle half of the hot fudge sauce over the batter.

Add the remaining brownie batter, then top with the remaining hot fudge sauce.

Use a spatula or knife to gently swirl the fudge into the batter.

Bake in the preheated oven for 30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.

Allow to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.