Make the crust: In a medium bowl, stir together the chocolate wafer crumbs and melted butter until evenly moistened. Press firmly into the bottom of a 9-inch springform pan. Freeze for 15 minutes while you prepare the filling.

Make the fudge ripple (optional): In a small saucepan, combine the heavy cream, sugar, cocoa powder, butter, and salt. Whisk over medium heat until smooth and bubbling, about 3 minutes. Remove from heat and let cool to room temperature, stirring occasionally.

In a large bowl, combine the sweetened condensed milk, vanilla, and salt. Whisk to blend.

In the bowl of a stand mixer fitted with the whisk attachment (or with a hand mixer), whip the cold heavy cream on medium-high speed until stiff peaks form—the cream should stand up firmly when you lift the whisk, but not look grainy or curdled. This takes about 3–4 minutes.

Gently fold one-third of the whipped cream into the condensed milk mixture to lighten it. Then fold in the remaining whipped cream until just combined—no white streaks remain, but do not overfold or the base will deflate.

If using, fold in the mini chocolate chips.

Spread half of the cream mixture over the chilled crust. Drop spoonfuls of half the cooled fudge ripple over the surface, then swirl with a knife. Repeat with the remaining cream mixture and fudge ripple, creating a marbled top.

Cover the pan tightly with plastic wrap and freeze for at least 6 hours, or overnight, until completely firm—the center should feel solid when pressed gently.

To serve, run a thin knife around the edge of the pan, release the springform side, and let the cake sit at room temperature for 5–10 minutes before slicing with a warm, dry knife. Wipe the blade clean between cuts for neat slices.