In a double boiler or a heatproof bowl set over simmering water, combine the chopped unsweetened chocolate, granulated sugar, and water. Stir gently until the chocolate has melted and the mixture is smooth. Remove from heat and let it cool slightly.
In a mixing bowl, whisk the egg yolks until they are pale and thick. Gradually add the melted chocolate mixture to the egg yolks, mixing until well combined. Stir in the Cointreau or Triple Sec if you’re using it.
In another clean mixing bowl, add the egg whites and cream of tartar. Using an electric mixer, beat the egg whites until they form stiff peaks.
Gently fold one-third of the whipped egg whites into the chocolate mixture to lighten it. Then, carefully fold in the remaining egg whites in two additions, being cautious not to deflate the mixture.
Fold in the finely ground chocolate wafer cookies until evenly distributed throughout the batter.
Pour the Chocolate Velvet batter into a greased 9-inch springform pan. Bake in a preheated oven at 350°F (175°C) for about 25-30 minutes or until the center is set but still slightly jiggly. Remove from the oven and cool completely in the pan.
Once cooled, refrigerate the Chocolate Velvet for at least 2 hours to set. When ready to serve, carefully remove the sides of the springform pan. Top with fresh whipped cream and enjoy!