Preheat the oven according to the brownie mix package directions and prepare the jelly roll pan with non-stick spray or parchment.
Mix both brownie packages with the eggs, oil, and water according to package instructions, then spread evenly into the prepared jelly roll pan and bake as directed.
Once baked, transfer the brownie pan to the refrigerator and cool completely, about 1 hour.
While the brownies cool, chop all but 6 to 8 of the Reese's miniatures and place in a large mixing bowl with 1 cup of the toffee bits.
Remove the ice cream from the freezer and let it sit at room temperature until just soft enough to scoop, about 10 to 15 minutes.
Scoop the softened ice cream into the bowl with the Reese's and toffee bits and fold together until evenly combined.
Place the springform pan upside down on the cooled brownie slab and press firmly to leave a circular impression, then cut along that outline and lift the brownie circle out cleanly.
Place the brownie circle into the bottom of the springform pan.
Spread the ice cream mixture over the brownie base in an even layer about 3 inches deep.
Crumble some of the remaining brownie scraps over the top of the ice cream layer.
Spread a thin layer of the remaining ice cream mixture over the crumbled brownie just enough to cover.
Sprinkle the reserved 1/4 cup of toffee bits evenly over the top.
Cover the pan tightly with tin foil and freeze until completely solid, at least 4 hours or overnight.
Thirty minutes before serving, remove the cake from the freezer to allow it to soften slightly.
Heat the hot fudge in a microwave-safe bowl in 15-second intervals, stirring between each, until poured easily.
Drizzle the hot fudge over the top of the cake, then arrange the reserved Reese's miniatures on top before serving.