How to Make It
In a medium bowl, using an electric mixer, beat the butter and confectioners' sugar at low speed until smooth. Beat in the egg and vanilla, then beat in the cocoa powder and salt. Add the flour and beat just until combined. Form the cookie dough into 2 disks, wrap in plastic and refrigerate for about 1 hour, or until firm.
Preheat the oven to 325°. On a lightly floured board, roll out each disk of chilled cookie dough to a 10 1/2-inch round, a scant 1/4-inch thick. Using a 2 1/2-inch round biscuit cutter, stamp out as many rounds as you can; transfer the rounds to a large rimmed baking sheet. Gather the dough scraps, chill briefly and reroll. (You should have 32 to 34 rounds.) Bake the chocolate cookies for 10 to 12 minutes, until they are just set. Let cool for 10 minutes, then transfer the chocolate cookies to a wire rack to cool completely.
Set a fine sieve over a medium bowl. In a medium saucepan, bring the half-and-half to a simmer and keep warm. In another medium bowl, whisk the egg yolks with the sugar, flour and salt until the mixture is pale, about 1 minute. Whisk half of the warm half-and-half into the egg yolks, then pour the mixture into the saucepan and cook over moderate heat, whisking constantly, until thickened, about 6 minutes. Remove the saucepan from the heat and whisk in the melted bittersweet chocolate and vanilla. Strain the pastry cream through the sieve and press a piece of plastic wrap directly onto the surface of the cream. Refrigerate the chocolate pastry cream for about 1 hour, or until chilled.
In a medium bowl, using an electric mixer, beat the cream with confectioners' sugar and the vanilla until firm.
Transfer 3/4 cup of the whipped cream to a small bowl. Beat in 3 tablespoons of the chocolate pastry cream until a light-chocolate cream forms.
Arrange 30 cookies on a work surface. Fill 3 pastry bags fitted with large plain tips with the chocolate pastry cream, the light-chocolate cream and the whipped cream. Pipe the chocolate pastry cream onto 12 cookies. Pipe the light-chocolate cream onto 6 cookies. Pipe the whipped cream onto the remaining 12 cookies. Stack the towers: Start with a chocolate pastry cream—topped cookie, then a light-chocolate cream—topped cookie, followed by a whipped cream—topped cookie, another chocolate pastry cream—topped cookie and ending with a whipped cream—topped cookie. Pipe a small dollop of any remaining chocolate pastry cream onto the top of each tower. Transfer the cookie towers to a serving platter and refrigerate for at least 6 hours before serving.