Step 1: In a large, chilled mixing bowl, combine the very cold heavy cream, powdered sugar, and vanilla extract. If making chocolate cream, also add the unsweetened cocoa powder.
Step 2: Using an electric mixer fitted with a whisk attachment, beat the cream mixture on medium-high speed until stiff peaks form. Be careful not to overbeat, or it will turn into butter.
Step 3: Spread a thin layer (about 1/4 inch thick) of the whipped cream mixture on the bottom of a 9x13 inch baking dish or an 8x8 inch dish for a thicker cake.
Step 4: Arrange a single layer of chocolate wafers or graham crackers over the cream, breaking cookies as needed to fit snugly.
Step 5: Spread another generous layer (about 1/2 inch thick) of whipped cream evenly over the cookies.
Step 6: Repeat the layering process: cookies, then cream, until all cookies and cream are used, ending with a final layer of cream on top. Aim for at least 4-5 layers.
Step 7: Cover the dish tightly with plastic wrap. Refrigerate for at least 6-8 hours, or preferably overnight, to allow the cookies to soften and create a cake-like texture.
Step 8: Before serving, garnish the cake with chocolate shavings, a dusting of cocoa powder, or fresh berries, if desired.
Step 9: Slice with a sharp knife and serve chilled. Enjoy your delicious no-bake chocolate icebox cake!