Place your large mixing bowl and the beaters from your electric mixer in the freezer for 10 minutes. In the chilled bowl, combine the 3 ½ cups very cold heavy cream, ½ cup powdered sugar, 1 teaspoon vanilla extract, and ¼ teaspoon salt. Beat on medium speed until soft peaks form, then increase to medium-high speed and beat until stiff peaks form. This takes about 3-5 minutes total. You'll know it's ready when you lift the beaters and the cream holds a peak that stands straight up and doesn't flop over. Be careful not to overbeat — if it starts to look grainy or curdled, you've gone too far and the cream will separate.
Spread a tiny dollop of cream on the bottom of your loaf pan (this acts as glue so the first layer doesn't slide around). Arrange a single layer of chocolate wafer cookies on the bottom. You'll likely need to break a few wafers in half to fit snugly — don't leave big gaps. Spread about ½ cup of whipped cream evenly over the cookies. Repeat this process: cookies, cream, cookies, cream. You should get about 6 to 7 layers total, depending on your pan. Finish with a final, generous layer of cream on top. Smooth it out with your spatula.
Cover the pan tightly with plastic wrap, pressing it directly onto the surface of the top cream layer (this prevents a skin from forming). Refrigerate for at least 8 hours. Overnight is best. Don't rush this step — the texture transformation is what makes the recipe work.
To serve, you can either slice it directly from the loaf pan or carefully invert it onto a serving plate. If inverting, run a thin knife around the edges first, place a plate on top, and flip. The cake should release cleanly. Garnish with chocolate curls, crushed cookies, or sprinkles before slicing.