Triple Chocolate Mousse Cake

MAKE THE CHOCOLATE SHORTBREAD CRUST

1. Pre-heat oven to 350 F. 
2. In a food processor (or using a pastry cutter, or two forks), process the flour, cocoa powder, sugar, and salt until combined.
3. Add butter and pulse (or cut) until crumbly, and the butter resembles small peas. 
4. Add egg and vanilla extract and pulse (or mix) until just incorporated.  It will still be a little crumbly, don't process until a ball forms. 
5. Line the bottom of an 8-inch springform pan with parchment paper and lock the side into place.
6. Transfer the dough into the pan and press it evenly across the base of the pan, on top of the parchment paper.  Prick the dough all over with a fork.
7. Bake the crust for 20 minutes and then transfer to a rack until completely cooled, about 30 minutes. 
8. Removed side of pan and remove the parchment paper from under the crust, and then reattach the side of pan.

MAKE THE CHOCOLATE MOUSSE

1. Sprinkle gelatin over 3 tbps of cold water in 1 qt. saucepan and let stand for 5 minutes.
2. Heat gelatin mixture over low heat, stirring, just until gelatin is melted, about 2 minutes.
3. Whisk in chocolate hazelnut spread until combined and then remove from heat.
4. Whisk mascarpone and chocolate hazelnut mixture in a large bowl. 
5. In a different large bowl, beat cream, cocoa powder, and sugar at low speed until just combined, and then increase speed to high and beat until cream just holds soft peaks.
6. Whisk ⅓ of whipped cream into the mascarpone mixture to lighten, then fold in remaining whipped cream until well combined.
7. Spoon filling onto shortbread crust in pan, smoothing top with a knife, or pastry knife. 
8. Chill, covered, for at least 3 hours.

MAKE THE GANACHE AND FINISH OFF THE CAKE

1. Bring cream to a simmer in a small saucepan and then remove from heat.
2. Add chocolate and let stand 1 minute, then gently whisk until smooth.
3. Transfer to a bowl and cool, stirring occasionally, until thickened, but pourable, about 20 minutes. 
4. Run a warm thin knife around inside of springform pan, then remove side.
5. Slide cake off the bottom of pan and transfer to a serving plate.
6. Pour ganache on top and spread, allowing excess to drip down the sides.