Heat the oven to 375 °F and line a sheet pan with parchment paper, then separate the egg. Keep the white in a completely clean bowl so it can whip to full volume.

Beat the egg white with the cream of tartar until stiff peaks stand upright, then set the meringue aside in another bowl. The peaks should look glossy and hold their shape rather than curl over at the tip.

Clean the mixing bowl, then beat the egg yolk, cream cheese, and stevia until smooth and pale yellow. The mixture should look creamy with no visible cream cheese lumps.

Fold the meringue into the yolk mixture with a light scoop-and-turn motion, then transfer the batter to a piping bag or snipped zip-top bag. Stop folding when the batter looks evenly blended and still feels airy.

Pipe about 10 round mounds, each about 2 inches wide, onto the prepared pan. Leave space between the mounds so heat can circulate and the puffs can expand.

Bake for 10 minutes, then lower the oven to 300 °F without opening the door and bake for 15 minutes more. The finished shells should be golden brown with a dry sheen and release easily from the parchment.

While the shells bake, whip the heavy cream, vanilla extract, and stevia until firm peaks form. Stop when the cream is thick and glossy, before it becomes grainy or starts turning buttery.

Cool the shells completely, split each one in half, and pipe in the whipped cream before replacing the tops. The finished puffs should feel light in the hand, with filling that stays neatly between the shells.