Preheat the oven to 350 degrees F with a rack in the center position.
Coat a 9-inch square baking pan with pan spray.
In a small bowl, whisk together the flour and salt; set aside.
Heat the oil in a medium saucepan over medium heat just until warm.
Use a whisk to stir in the sugar until it is evenly moistened.
Add the yogurt and vanilla, and stir until thoroughly combined.
Stir in the cocoa powder, several tablespoons at a time, to make a thick, dark paste.
In a small clean bowl and using a clean whisk, beat the egg whites just until they are very thick and foamy.
Whisk the whites into the chocolate mixture just to combine.
Fold in the flour mixture until streaky, then fold in the chocolate chips just until combined.
Spread the batter evenly in the prepared pan.
Bake for 15 minutes, or until the top has formed a crust and a knife inserted into the center still comes out with wet crumbs on it.
Transfer the pan to a wire rack and it let cool for at least 1 hour before cutting.
To serve: Cut the brownies into 4 even strips one way, then 4 the other, to make 16 brownies.
Store any leftover brownies at room temperature, tightly wrapped in plastic film, for up to 4 days, or overwrap with foil and freeze for up to 2 weeks.
Thaw individual frozen brownies at room temperature.