Spray the inside of your slow cooker with oil to prevent sticking. In a medium bowl, combine the brownie mix, oil, 1/4 cup water, eggs, and salt. Stir until just combined—you want a thick, uniform batter without overmixing. I like to use room temperature eggs here because they blend more smoothly with the other ingredients, creating a better texture in the finished brownie.

Spread the brownie batter evenly into the prepared slow cooker. In a small bowl or measuring cup, whisk together the fudge sauce and 1 cup of boiling hot water until well combined and pourable. The hot water is essential here—it creates steam that helps the brownies rise while also forming a rich, fudgy sauce layer beneath the brownie cake. Pour this mixture slowly and evenly over the top of the batter, being careful not to stir or disturb the batter underneath.

Cover the slow cooker and cook on HIGH for 2 1/2 to 3 hours. The brownies are done when the top looks set and a toothpick inserted into the cake portion (not the sauce below) comes out with just a few moist crumbs—you want it fudgy, not dry. I find that 2 hours 45 minutes is my sweet spot, but slow cookers vary, so check at 2 1/2 hours. Serve warm directly from the slow cooker with vanilla ice cream on top, allowing the hot fudge sauce to pour over the ice cream for an incredible warm-and-cold contrast.