1: Melt 100 g butter in a small saucepan over medium heat, then continue cooking until it no longer boils and gives off a nutty smell, you have made a "nutty" butter. Let it cool off the heat until it is lukewarm.
5: Add the warm hazelnut butter, whisking gently...
6: ...until the mixture is homogeneous.
7: The madeleine dough is finished, but it is important to let it rest in the cold before using it. The best way to do this is to pour it into a pastry bag, which is itself placed in a high-sided container.
8: Close the piping bag and refrigerate for at least 4 hours, ideally overnight.
9: Preheat your oven to 180°C (360°F). Butter a madeleine tray, then pipe a small pile of dough into each mould. Don't overfill, otherwise your madeleines will stick together when they are baked, as they will swell up a lot.
10: Bake in the oven for 15-20 minutes, watching the coloring at the end.
At step 7 you can also simply put the dough in the bowl in the fridge overnight, then pipe or spoon it the next day. You don't have to make a hazelnut butter, a simple melted butter will work too, but it won't taste as good. If you don't have vanilla sugar, replace with the same weight of sugar and a teaspoon of vanilla extract.