2) In a small saucepan over medium heat, combine the water, butter and salt and bring to a boil. Reduce the heat, add the flour and cinnamon all at once, stirring vigorously with a wooden spoon. Cook until the mixture forms a ball, has a slightly sweaty sheen to it, and has pulled away from the sides of the pan.
3) Transfer the mixture to a bowl and allow to cool for 3 to 4 minutes. It does not have to be cold, just cool enough so the eggs don't cook when you add them. Using an electric mixer or lots of old fashioned elbow grease, beat in the eggs 1 at a time. Do not add the second egg until the first is fully incorporated.
4) Transfer the mixture to a pastry bag equipped with a large straight tip and pipe 2.5cm balls onto a baking tray lined with greaseproof paper. Be sure to leave at least 2.5cm between each ball since they grow as they cook! When done, dip your finger in water and smooth the top of each ball where the pastry tip was removed.
5) Bake for 20 to 25 minutes, rotating the tray halfway through the cooking time to ensure even browning. The puffs should be light, airy, and dry inside. Cool on a rack.
For the mascarpone filling:
1) Put the double cream into the cold bowl and whip it to stiff peaks with an electric mixer. Put the mascarpone in a large bowl and gently mix in the praline paste. When the praline paste is combined, begin to fold in the whipped cream.
2) When the whipped cream is fully incorporated, spoon the mixture into a clean pastry bag. Cut the top off each profiterole and fill with the mascarpone mixture. Replace the tops and drizzle with warm chocolate sauce. Transfer to a large serving platter and garnish with chopped hazelnuts.
For the chocolate sauce:
1) Fill a medium saucepan with 5cm of water and put it over medium heat. In a bowl large enough to sit on the saucepan without the bottom touching the water, add the chocolate, cream, butter, corn syrup and cinnamon.
2) Stir until the chocolate has melted and everything is combined. This is a pretty quick process. When the chocolate is melted, remove it from the heat, and spoon it over the filled profiteroles. This is best served warm!