ESPRESSO CAKE:
1/4 cup ground espresso
1/2 cup water
1 cup cake flour (not self rising)
1/2 teaspoon baking soda
1/4 teaspoon salt
4 tablespoons unsalted butter
1/2 cup + 2 tbl. sugar
2 large eggs
1/2 teaspoon vanilla
1/4 cup sour cream

CHOCOLATE CHUNK MOUSSE:
5 ounces bittersweet chocolate, coarse chopped
1/4 cup + 2 tbl milk
1/4 cup sugar
1/8 teaspoon salt
2 teaspoons vanilla
1/2 cup Bailey's irish Cream
6 ounces mascarpone cheese, softened
1 cup heavy cream
1/4 cup hazelnuts, toasted, coarse chopped
4 ounces bittersweet chocolate, cut in chunks

GARNISH:
3/4 cup heavy cream
2 tablespoons Irish cream liqueur
1 tablespoon powdered sugar
2 ounces bittersweet chocolate, cut up

ESPRESSO CAKE: Position a rack in the lower third of the oven and preheat to 350ºF. Lightly butter an 8-inch square cake pan. Dust the pan with flour and tap out the excess. Place the ground espresso into a cup or small bowl. Heat the water to boiling and pour it over the espresso powder. Allow the grounds to steep for 5 minutes. Strain the coffee through a double layer of cheesecloth. Measure 1/4 cup of the coffee and set aside. In a large howl, using a wire whisk, stir together the flour, baking soda and salt. Sift the mixture onto a large piece of waxed paper. In the 4 1/2-quart bowl of a heavy-duty electric mixer using the paddle attachment, beat the butter on medium speed for 1 to 2 minutes until creamy.

Gradually add the sugar, blending well between additions, and scraping down the sides of the bowl when necessary. Add the eggs one at a time and beat until combined. Add vanilla and sour cream. On low speed, add half of the flour mixture and beat until combined. Add the 1/4 cup of espresso, and mix well. Add the remaining flour mixture. Scrape the batter into the prepared pan and bake for 25 to 30 minutes or until the cake begins to pull away from the sides of the pan and a tester inserted into the center of the cake comes out clean. Remove pan from oven and set on a wire rack to cool completely.

CHOCOLATE CHUNK MOUSSE: Put the chocolate in a food processor fitted with the metal chopping blade. Process for 20 to 30 seconds, until finely ground. In a small saucepan, combine the milk, sugar and salt. Cook over medium heat, stirring with a wooden spoon, until the sugar dissolves and the milk comes to a boil. Remove the pan from the heat. Add the vanilla extract and 1/4 cup of the Bailey's. With the motor of the food processor running, pour the hot milk through the feed tube. Process for 10 to 20 seconds, until the chocolate is completely melted. Using a spatula, scrape the chocolate mixture into a large bowl and cool for about 5 minutes, until tepid. In the 4 l/2-quart bowl of a heavy-duty electric mixer using the paddle attachment, beat the mascarpone at medium-low speed just until softened.

Gradually add the rest of the Bailey's, scraping down the side of the bowl as necessary. Switch to the wire whip attachment, and beating on medium speed, add the heavy cream. Increase the speed to medium-high and continue beating 2-3 minutes, until soft peaks are formed when the whip is lifted. Using a large rubber spatula, fold one-third of the whipped cream mixture into the chocolate mixture to lighten it. Fold the remaining whipped cream into the chocolate mixture. Fold the toasted nuts and chocolate chunks into the mousse.

ASSEMBLE THE CAKE: Line an 8-inch square pan with foil, leaving a 2-inch overhang on two opposite sides of the pan. Using a long serrated knife, slice the cake horizontally into two layers of equal thickness. Place the top layer, cut side up, in the bottom of the pan. Scrape the mousse onto the cake layer in the pan. Smooth the top with a small metal cake spatula. Place the second layer, cut side down, on top of the mousse. In the 4 1/2-quart bowl of a heavy-duty electric mixer using the wire whip attachment, combine the heavy cream, Bailey's, and confectioners' sugar and beat on medium-high speed until medium-stiff peaks are formed when the whip is lifted. With a small metal spatula, spread the top of the cake with the whipped cream. Sprinkle the chocolate chunks over the whipped cream. Freeze the cake for 6 hours or overnight.

Remove the cake from the freezer. Lift it out of the pan, using the overhanging pieces of foil as handles, and set it on a cutting board to temper for 30 minutes. With a sharp knife, trim the four sides of the cake, then cut into eight 4-inch-by2-inch bars. Serve on dessert plates with warm chocolate sauce.