Line a 9-inch springform pan with plastic wrap, leaving overhang. This saves cleanup.
Press half the brownies into the bottom in an even layer.
Spread half the ice cream over brownies. Smooth with an offset spatula.
Drizzle half the hot fudge and sprinkle nuts or chips if using.
Add remaining brownies in a single layer, then top with the rest of the ice cream.
Freeze 4 hours or overnight until firm. Run a warm knife around edge to release.
Remove plastic wrap, spread whipped cream, drizzle remaining fudge, slice and serve.