Step 1

Preheat oven to 300 degrees F. Butter or spray a 10-inch springform pan; line pan with parchment paper and butter or spray parchment.

Step 2

Pulse cookies in a food processor to very fine crumbs. Transfer to a bowl, then add melted butter and stir until well combined.

Step 3

Pour crust mixture into prepared pan, pressing evenly with the back of a spoon into bottom and up sides, leaving about ½ inch between the top of the crust and top of the pan. Transfer to freezer 10 minutes, until crust is set.

Step 4

Transfer pan to oven and bake 10 minutes, until dry to the touch. Allow to cool. Increase oven temperature to 350 degrees.

Step 5

Place butter and chocolate in a microwave-safe bowl (or set over double boiler). Heat in 30-second increments until melted, then stir to combine. Whisk in whiskey (or water), cocoa powder, vanilla, and salt; set aside.

Step 6

In the bowl of an electric mixer fitted with a whisk attachment, beat egg yolks with brown sugar for about 5 minutes, until light and almost doubled in volume. Add chocolate-whiskey mixture and beat until just combined, scraping down sides and bottom of the bowl as necessary.

Step 7

In a separate, clean bowl of an electric mixer fitted with the whisk attachment, beat egg whites until foamy. Gradually increase speed to high and slowly add granulated sugar, beating until soft peaks form.

Step 8

Transfer 1 cup egg white mixture to chocolate mixture and gently fold in with a spatula to combine. Add remaining egg whites and continue gently folding until they are almost completely combined; do not overmix.

Step 9

Pour into cooled crust and bake 35-40 minutes at 350, until cake edges are set but still jiggles slightly in the center. Transfer to a wire rack to cool completely. Wrap with plastic wrap and refrigerate 3-24 hours.

Step 10

In a medium saucepan, whisk together sugar, cocoa powder, cornstarch, espresso powder, and salt. Add egg yolks and whisk until combined, creating a thick paste. Slowly add milk, whisking constantly.

Step 11

Set saucepan over medium heat and bring mixture to a boil, whisking constantly. Boil for 30 seconds (mixture should begin to thicken,) then immediately remove from heat. Add butter, chopped chocolate, and vanilla, whisking until combined.

Step 12

Let stand at room temperature for 15 minutes, then press plastic wrap directly on the surface of pudding to prevent a skin from forming. Transfer to refrigerator until chilled, about 3 hours. Using an offset spatula, spread pudding to form an even layer on top of the cooled cake. Transfer back to refrigerator for 30 minutes.

Step 13

Chocolate Irish Cream MousseIn a microwave safe bowl, melt chocolate and ½ cup heavy cream in 30-second intervals. Stir to combine, adding sugar and Irish cream; allow to cool to room temperature.

Step 14

In the bowl of an electric mixer fitted with a whisk attachment, beat cream until soft peaks form, about 1 minute. Beat in chocolate mixture ⅓ at a time, until fluffy.

Step 15

Spread on top of chilled pudding layer, then return to refrigerator for at least 1 hour.

Step 16

In the bowl of an electric mixer fitted with a whisk attachment, beat cream until soft peaks form, about 1 minute. Sprinkle sugar over cream and slowly drizzle in Irish cream, whisking until stiff peaks form.

Step 17

Spread whipped cream over chilled cake. Garnish with shaved chocolate. Refrigerate for 30 minutes minimum, then unmold cake and serve immediately.