Preheat the oven to 400F (204C) and move a rack into the top third of the oven. Line several baking sheets with parchment paper. In a small bowl, whisk together the egg yolk and heavy whipping cream and set aside.
Place the milk, water, butter and sea salt in a medium, heavy-bottom saucepan over medium heat. Stir frequently until the butter melts and then turn up the heat to medium-high and you start to see bubbles around the exterior of the pan. Dump all of the flour into the saucepan and stir rapidly with a wooden spoon until the dough starts to pull away from the sides of the pan and a mass forms.
Scrape the mixture into the bowl of an electric stand mixer fitted with the paddle attachment and let the mixture cool for a couple of minutes. With the mixer on low add in the eggs one at a time, making sure that one is well blended before adding in the next. Scrape the sides and bottom of the bowl to make sure everything is combined.
Use a 1 1/2 tablespoon cookie scoop (or estimate) to heap mounds onto the prepared baking sheets, leaving 1 1/2 inches between mounds. With a damp (not dripping wet) finger smooth over the tops. Brush the tops with the egg-cream wash and sprinkle each one with a pinch of parmesan.
Bake in the top third of the oven for 23-25 minutes or until golden brown. Once out of the oven, pierce the side of each cheese puff with a small knife to release some steam.