Pour the eggs and sugar into the mixture, then return to the heat.
A custard is cooked to 89°C. It should be vanilla-coated, not whipped.
To do this, use a wooden spoon to stir the cream back and forth in the pan.
Once the temperature reaches 89°C, strain the cream and leave to cool at room temperature.
Now that everything is cold, churn the custard in an ice cream maker until it becomes thick and frozen.
Transfer to a bowl and add the praline paste.
Let the butter soften at room temperature. In a mixing bowl, cream together with the sugar.
Add the egg and vanilla sugar, if desired.
Coarsely chop the hazelnuts.
Gradually add the flour, baking powder, salt, and hazelnuts. Mix.
Butter a baking sheet or cover it with a silicone mat. Using two tablespoons or simply your hands, form balls of dough, spacing them apart as they will spread during baking.
Bake for 8 to 10 minutes at 180°C or gas mark 6. Remove them as soon as the edges start to brown.
Remove the ice cream from the freezer 10 to 15 minutes before serving.
Spread a generous layer of ice cream on 6 cookies and sandwich them together with the remaining cookies. Press the cookies together slightly, then smooth off any ice cream that may have spilled over.
