The Greek yogurt kind of "melts" in the oven this is why I suggest pressing down on the brownie batter before swirling in the Greek yogurt so that you still get pretty swirls in your brownies. If you don't the brownies will still be delicious though!
The Greek yogurt swirl originally made enough to make a 9x13 pan so I halved it for this recipe which is why there is half an egg. One large egg should be 4 tbsp or 1/4 C for half an egg, beat the yolk and white together and measure out 2 tbsp. You can freeze the rest of the egg for another use or save it in the fridge for the next time you make an omelet or scrambled eggs.