Preheat your oven to 350°F (177°C). Spray the inside of a 7-inch round cake pan with cooking oil spray. Line the bottom and sides of the pan with parchment paper, leaving handles for easy removal of the cake once baked.
Separate the egg yolks from the egg whites. Place the egg whites in the fridge until ready to use.
In a large mixing bowl, whisk together the egg yolks, yogurt, and sweetened condensed milk until smooth and fully incorporated.
Remove the egg whites from the fridge and beat them in a separate large mixing bowl at the highest speed until stiff peaks form. A stand mixer works best, but a hand mixer will also do the job.
Gently fold one-third of the beaten egg whites into the egg yolk mixture using a spatula. Continue folding in the remaining egg whites in two more portions, being careful to incorporate the batter fully from the bottom of the bowl.
Pour the light and airy batter into the prepared cake pan.
Prepare a water bath by placing the cake pan into a larger baking dish (at least 2 inches deep). Pour cold tap water into the outer pan until the water level reaches 1/2 inch.
Bake the cake in the preheated oven for 40 minutes. Then, reduce the oven temperature to 325°F (163°C) and continue baking for 10 more minutes, or until the cake has risen and the surface is dark brown.
Turn off the oven but leave the cake inside with the door closed for one hour. This helps the cake finish cooking and prevents sudden collapse due to a temperature drop.
After one hour, remove the cake from the oven and use the parchment paper handles to lift it from the pan. Let the cake cool to room temperature, then refrigerate (uncovered) for several hours or overnight.
Before serving, you can dust the top with powdered sugar if desired. Serve cold or at room temperature.