Preheat the oven to 425 degrees F and line 2 baking sheets with parchment.

Place the water, butter, sugar, and salt in a large saucepan and cook over medium-high heat until the butter is completely melted and the mixture is simmering.

Add the flour all at once and continue to cook, stirring, for about 5 minutes or until the dough gathers into a ball.

Transfer the mixture to a mixing bowl and beat on medium speed, adding the eggs one at a time until fully incorporated.

Pipe or spoon the dough onto parchment-lined baking sheets, allowing about 3 inches between each puff.

Bake for 25-35 minutes or until very dry looking, puffed, and deep golden brown.

For the cream filling, whisk together the yolks, sugar, cornstarch, and salt in a medium bowl.

Heat the milk in a small pot until small bubbles form around the edge.

Gradually add the hot milk to the egg mixture, whisking to combine.

Transfer the mixture back to the pot and cook over medium-low heat until thickened, then stir in the vanilla.

Pour the pastry cream through a fine mesh strainer into a heat-safe bowl and press a layer of plastic wrap directly onto the surface. Refrigerate until completely cool (about 2 hours).

Whip the cream until it holds stiff peaks and fold it into the cooled pastry cream.

For the toppings, melt the butter in a large skillet over medium heat, then add the apples, sugar, cinnamon, and salt.

Cook, stirring occasionally, until the apples have softened slightly (about 5 to 10 minutes).

Slice the baked choux puffs in half horizontally and layer the bottom half with 5 to 8 slices of cooked apples and about 1/4 cup of cream filling.

Sandwich with the top half of the puff and drizzle with salted caramel sauce.