You’ll never guess the secret ingredient in this lush Portuguese custard:
15 large egg yolksPrepare the caramel: In a medium pot set over high heat, stir together the sugar and 3 tablespoons of water. Bring to a boil and cook, without stirring, until the caramel is a deep amber color, 10–12 minutes. Remove from the heat, and carefully pour the caramel into a 9-inch savarin mold or tube cake pan, tilting the pan and using the back of a silicone spatula as needed to evenly coat the bottom of the mold. Set aside to set.
Make the custard: To the same pot, add the sugar, lemon zest, cinnamon stick, and 2¼ cups water and bring to a boil over medium heat. Add the presunto and cook, stirring continuously until the sugar dissolves. Remove from the heat and set aside to infuse and cool for 15 minutes. Position a fine mesh strainer over a large liquid measuring cup and strain the sugar syrup; discard the ham solids and cinnamon stick.
In a large bowl, whisk together the egg yolks, egg whites, and the Port. Gradually whisk into the strained sugar syrup until smooth, then pour the custard into the prepared mold. Transfer the mold to a large baking dish, then fill the baking dish with enough hot water to come halfway up the outside of the custard mold. Transfer to the oven and bake until the custard set, with a slight jiggle when shaken, about 1½ hours. Remove from the oven and cool the custard to room temperature.
To serve, run a thin, offset spatula or a paring knife along the edges of the mold, then place a large, rimmed serving platter upside-down atop it. Quickly and confidently, invert the mold and the platter together to unmold the pudding. Remove the mold, allowing the caramel to run down the sides of the custard. Cut into slices and serve chilled.