Step 1: Prepare the chocolate cake mix according to package directions. Bake in a 9x13 inch baking pan or two 9-inch round cake pans. Let the cake cool completely.
Step 2: If using a 9x13 inch pan, carefully slice the cake horizontally into two even layers. If using two 9-inch round pans, level the tops if necessary.
Step 3: Place one layer of cooled chocolate cake in the bottom of your serving dish or springform pan.
Step 4: Spread half of the slightly softened vanilla ice cream evenly over the first cake layer. Drizzle generously with half of the warmed hot fudge sauce.
Step 5: Add the second cake layer on top of the fudge and ice cream. Spread the remaining ice cream evenly over the second cake layer, then drizzle with the remaining hot fudge sauce.
Step 6: Cover the cake tightly with plastic wrap and freeze for at least 4-6 hours, or overnight, until firm.
Step 7: Before serving, prepare fresh whipped cream by beating heavy cream with powdered sugar and vanilla until stiff peaks form. Spread whipped cream over the top of the frozen cake.
Step 8: Garnish with chocolate shavings or sprinkles, if desired. Let the cake sit at room temperature for 5-10 minutes before slicing and serving for easier cutting.