Step 1: Line an 8×8 inch square baking dish or a 9×5 inch loaf pan with parchment paper, leaving an overhang on two sides for easy removal.
Step 2: Place a large mixing bowl (preferably metal) and the whisk attachment of an electric mixer in the freezer for 10-15 minutes to chill thoroughly.
Step 3: In the chilled bowl, combine the very cold heavy cream, powdered sugar, unsweetened cocoa powder, and vanilla extract. Beat on medium-high speed with the electric mixer until stiff peaks form. Be careful not to overbeat.
Step 4: Spread a thin layer of the chocolate whipped cream (about 1/4 inch thick) on the bottom of the prepared pan.
Step 5: Arrange a single layer of chocolate wafers over the cream, breaking them as needed to fit snugly and cover the cream layer completely.
Step 6: Spread another generous layer of chocolate whipped cream over the cookies, gently pressing down to ensure even coverage. Repeat the layering process, alternating between cookies and cream, until all ingredients are used, finishing with a final layer of whipped cream on top. You should have approximately 4-5 layers of cookies.
Step 7: Cover the pan tightly with plastic wrap and refrigerate for at least 6 hours, or preferably overnight, to allow the cookies to soften and the flavors to meld.
Step 8: Once thoroughly chilled, use the parchment paper overhangs to carefully lift the cake out of the pan. Gently peel away the parchment paper. Garnish with chocolate shavings, fresh berries, or a dusting of cocoa powder, if desired. Slice and serve immediately. Store any leftovers in an airtight container in the refrigerator.