Combine the Granulated Sugar and Water in a small saucepan over medium heat, stirring until the sugar dissolves; then bring to a gentle boil and cook until the syrup thickens and becomes glossy (it should coat the back of a spoon).
Remove the pan from heat and let the syrup cool for a minute or two so it’s hot but not scalding.
Place the Egg Yolks in a heatproof bowl and whisk lightly to break them up.
Temper the yolks by gradually adding a few spoonfuls of the hot syrup into the yolks while whisking constantly; then slowly pour the tempered yolk mixture back into the saucepan with the remaining syrup.
Return the pan to very low heat and cook gently, stirring constantly with a wooden spoon or spatula, until the mixture thickens to a smooth, creamy custard that coats the spoon—do not let it boil to avoid curdling.
Remove from heat and, if desired, pass the custard through a fine sieve for extra smoothness. Allow it to cool slightly before handling.
Spoon or pipe the warm custard into the crisp Hóstias de Aveiro wafers, filling each shell generously but neatly.
Let the filled ovos moles cool to room temperature so they set a little, then refrigerate briefly if you prefer them chilled. Serve gently on a plate and enjoy within a day or two for best texture.
