Prince William broke with royal tradition by having a groom's cake at his wedding to Kate Middleton in 2011, requesting a classic British icebox cake (or fridge cake, as the Brits say). It's made out of McVitie's biscuits and lots of chocolate. The cake is also said to be a favorite of his grandmother Queen Elizabeth, who likes eating a slice with her afternoon tea.
Make the cake: Preheat the oven to 350 degrees F. Spread the hazelnuts on a rimmed baking sheet and toast in the oven, stirring occasionally, until golden, about 10 minutes. Let cool, then chop.
Butter the bottom and sides of an 8-inch springform pan; set aside. Toss the hazelnuts and cookies in a large bowl. Combine the butter, golden syrup and salt in a medium saucepan over medium heat and cook, stirring, until the butter is melted (do not boil). Remove from the heat and stir in the chocolate, cocoa powder and vanilla until smooth, then whisk in the chocolate-hazelnut spread.
Pour the chocolate mixture over the cookie mixture and stir to coat. Transfer to the prepared pan and press down evenly until tightly packed. Place a piece of plastic wrap directly onto the surface and refrigerate until set, at least 6 hours or overnight.
Make the ganache: Put the chocolate in a small bowl. Heat the heavy cream and golden syrup in a small saucepan over medium heat, stirring occasionally, until the syrup dissolves and the mixture is just bubbling around the edges. Pour over the chocolate and let stand 5 minutes, then stir until smooth. Let sit at room temperature until thickened but still pourable, 30 minutes to 1 hour. (It should look like a soft pudding.)
Remove the cake from the refrigerator and let sit at room temperature, 45 minutes. Remove the springform ring and invert the cake onto a cake stand or serving plate; remove the bottom of the pan. Pour about one-third of the cooled ganache over the top of the cake and gently spread with a small offset spatula, allowing the excess to drip down the sides. Repeat with the remaining ganache in two more batches until the top and sides of the cake are covered. Sprinkle mini chocolate chips around the top edge of the cake.