Heat your oven to 325°F and line the bottom of an 8x8-inch pan with parchment paper, spraying the sides with cooking spray.
Fill a medium pot half way with water and bring to a boil. Place the butter, oil and sugar in a glass or aluminum bowl that fits in the top of the pot to create a double boiler.
Once the water boils, place the bowl on top and whisk until the butter is melted and everything forms a grainy mixture.
Remove from the heat and stir in the vanilla and yogurt.
Stir in the flour, cocoa powder, cornstarch, baking soda and salt JUST until mixed.
Finally, stir in the hot water. Once it's mixed in, stir in the chocolate chips. Stir the least amount possible or the brownies might sink after cooking.
Pour into the pan and bake until the top is set and slightly crackly, about 35-40 minutes.
Immediately place in the fridge on a hot pad for 10 minutes.
Remove from the fridge and let cool COMPLETELY before slicing.