Preheat the oven to 350°. Line the bottom of two 8-inch cake pans with parchment paper, grease, and flour the sides. (The parchment paper will make the cake come out much easier than just grease and flour).

In a medium bowl, whisk the hot coffee and cocoa together until smooth. Set aside.

In a large mixing bowl, add the sugar, flour, baking powder, baking soda, and salt. Using a whisk, blend and sift the ingredients together.

Add the eggs, milk, oil, vanilla, and the cocoa mixture to the dry ingredients. Mix on low speed for 30 seconds until the dry ingredients are moist. Increase the speed to medium and beat for 1 minute. Scrape down the sides and beat for another minute. The batter will be thin.

Pour the batter into the two prepared pans and place in the center of the preheated oven. Bake for 33 to 37 minutes (a toothpick inserted in the center of the cakes should come out clean).

Let cool in the cake pans for 5 to 10 minutes then run a knife around the edge of the pans to loosen the cake from the pans. Invert the cakes onto a cooling rack and remove the parchment paper.

Add the chocolate and butter to the top of a double boiler which is set over barely simmering water and allow to melt undisturbed.

While the chocolate is melting, sprinkle the gelatin into 1/4 cup of cold water and set aside for 1-2 minutes to bloom.

Pour the hot coffee into a small dish and add the espresso powder, stirring to dissolve.  Add the softened gelatin, stir to combine with the espresso and set aside to cool. 

In a large mixing bowl beat the egg yolks until they are thick and pale. Beat in the cooled espresso mixture and the Irish whiskey, if using.  Add the melted chocolate by spoonfuls, beating until the mixture is combined well.

Stir one-third of the meringue into the chocolate mixture then fold in the remaining meringue.

In a chilled bowl beat the heavy cream until it holds soft peaks, beat in the remaining ¼ cup of sugar, and beat the cream until it holds stiff peaks.

Fold the whipped cream into the chocolate mixture, gently incorporating it completely.

Place one cake layer on a plate and spread the mousse in a thick layer, add the second cake layer then refrigerate the cake for 6 hours or overnight allowing the mousse to set.  Then proceed with the whip cream frosting.

In a large chilled bowl stir the cream and the confectioners’ sugar together.  Attach a whisk to the mixer and adjust the speed to low. 

Whip on low until the cream begins to thicken and doesn’t spatter.  Increase the speed to medium-high and whip until the cream holds soft peaks. 

Stop the motor and add the sour cream, then begin whipping on low speed while gradually increasing the speed to medium-high.  Whip on medium-high until the cream holds stiff peaks.