Preheat the oven to 300 degrees F (149 degrees C). Line a baking sheet with parchment paper and grease lightly.
In a large bowl, use an electric mixer to beat the egg whites and cream of tartar, until stiff peaks form. (Make sure they are stiff peaks, not soft peaks!)
In a second large bowl, use the mixer to beat the mascarpone, egg yolks, and sea salt until smooth.
Carefully and gradually fold the egg whites into the mascarpone mixture with a spatula. Use a folding motion to incorporate, being careful not to break down the air bubbles in the whites.
Scoop the mixture into 6 circular discs onto the parchment paper.
Bake for 25-35 minutes, until golden.
Let the cloud bread cool a bit on the baking sheet, then transfer to a cooling rack to cool completely before enjoying.