Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan.

In a large mixing bowl, combine the brownie mix, vegetable oil, eggs, and water. Stir until well blended.

Pour the brownie batter into the prepared cake pan and spread it evenly. Bake for 20-25 minutes or until a toothpick inserted in the center comes out clean. Let the brownie cool completely in the pan.

Once the brownie is cool, carefully remove it from the pan and place it on a serving plate. Slice the brownie in half horizontally to create two layers.

Place one layer of brownie back on the serving plate. Spread half of the softened vanilla ice cream over the brownie layer. Drizzle half of the hot fudge sauce over the ice cream.

Place the second layer of brownie on top of the ice cream. Spread the remaining ice cream over the top layer and drizzle with the remaining hot fudge sauce.

Top the cake with whipped cream and sprinkle with chocolate shavings or sprinkles if desired. Freeze the cake for at least 2 hours to set.

Before serving, let the cake sit at room temperature for about 10 minutes to soften slightly. Slice and enjoy!