Preheat oven to 400°F (200°C) and line two baking sheets with parchment.

Fit a pastry bag with a plain 1/2-inch round tip (or use a zip-top bag with corner snipped).

In a medium saucepan, combine water, milk, butter, sugar, and salt. Bring to a rolling simmer.

Remove from heat and add all the flour at once, stirring vigorously until the mixture pulls away from the pan and forms a smooth ball.

Return the pan to low heat and stir for 1–2 minutes to dry the dough slightly — you’ll see a thin film form on the bottom.

Transfer dough to a mixing bowl (or stand mixer). Beat in eggs one at a time, ensuring each is fully incorporated before adding the next; the dough should be glossy and just hold its shape on a spoon.

Pipe small rounds (about 1½ inches) onto prepared sheets, spaced 1–2 inches apart. Smooth any peaks with a wet fingertip.

Bake at 400°F for 15 minutes, then reduce heat to 350°F (175°C) and bake 12–15 more minutes until puffed and deep golden. Do not open the oven during baking.

Pierce each puff with a skewer to release steam and let cool on a rack.

Heat cream until just simmering, pour over chopped chocolate and hazelnut spread in a bowl. Let sit 1–2 minutes, then whisk until smooth.

Stir in butter if using and chill until thick enough to pipe (about 30–45 minutes).

Heat cream and corn syrup until just simmering, pour over chopped chocolate, let sit, then whisk smooth.

Keep warm slightly to glaze.

Slice or make a small hole in the base of each puff. Pipe in the chocolate-hazelnut filling.

Dip the tops into warm ganache or spoon it over for a rustic drip.

While the ganache is still tacky, gently brush or dust tops with edible gold powder or press a minuscule piece of gold leaf.

Sprinkle nuts if using.