Preheat oven to 400F (200C), and line two eight-inch pans with parchment paper.

Pour the water into a medium saucepan. Add the butter, salt, and sugar, and cook over medium heat until the butter has melted and the water is boiling.

Add the flour into the water mixture and, over medium heat, use a wooden spoon to stir the mixture until you have a smooth dough ball. About 2-3 minutes.

Remove from heat and transfer the dough into a stand mixer bowl. Use the paddle attachment to mix the dough on medium-low and cool it down to room temperature.

When the dough has cooled down, on high speed, add the eggs one at a time, waiting for each egg to incorporate before adding the next.

Divide the dough between the two prepared pans (about 315g each), and bake for 15 minutes, then reduce the heat to 350F and bake for an additional 15 minutes.

Turn the oven off and let the cake cool inside the oven, door shut, for 10-15 minutes.

Remove the cake from the oven and allow it to cool completely before filling.

Line the sides of one eight-inch springform pan or a pan with a removable bottom with a 5-inch wide piece of parchment.

Line one cake layer at the bottom of the pan.

Evenly spread the mousseline cream then top it with the second layer.

Refrigerate for a minimum of one hour and allow it to set.

When ready to serve, invert the cake onto your serving plate and serve.