Preheat the oven to 350°F. Grease an 8-inch by 8-inch metal baking pan with cooking spray or oil, then line with a parchment paper sling.
Place 1.5 ounces bittersweet chocolate and 113 grams unsalted butter in a small bowl and heat in the microwave in 15-second intervals until melted. Whisk until smooth and homogeneous.
Whisk in 24 grams dutch-process cocoa powder and 1 teaspoon instant espresso. Allow to cool slightly until lukewarm but not hot.
In a medium bowl, whisk 100 grams eggs and 267 grams granulated white sugar together until light and frothy, about 2 minutes. Add the chocolate mixture to the egg mixture and stir to combine.
Mix in 4 ½ ounces greek yogurt or skyr and 6 grams pure vanilla extract.
Fold in 92 grams all-purpose flour and ½ teaspoon kosher salt until just barely combined. Fold in 80 grams semi-sweet chocolate chips.
Transfer the batter to the baking pan and smooth it out in an even layer with a butter knife or offset spatula.
Bake in the oven for 30 - 35 minutes, or until a toothpick inserted into the center of the pan comes back with just a few moist crumbs. Top with flaky salt if desired.
Remove from the oven and cool for 10 minutes. Gently remove the brownies from the pan using the parchment paper sling and transfer to a wire rack to cool completely.For a fudgier brownie, chill the cooled brownies in the fridge for an additional 2 - 4 hours. Remove the brownies from the fridge, slice them into equal portions, then allow to thaw to room temperature before serving.
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Notes
I always recommend using a kitchen scale for baking for the most accurate results. But if you are using cup and tablespoon measurements, make sure to use the scoop and level method for measuring cocoa powder and flour!
FlavoredGreek Yogurt: I love using a coffee flavored greek yogurt (or skyr) here. You only need 4.5 ounces, which is about the size of a single serve container. If you can’t find a coffee flavor, try a vanilla yogurt. Avoid using a yogurt that’s overly sweet (more than 15 grams per 4.5 ounce serving).
Cooled and sliced brownies can be stored in an airtight container at room temperature for 3 - 4 days. You can also store them in the fridge for up to a week.
Freeze cut brownies in an airtight plastic bag in the freezer for 1 - 2 months. You can reheat them in the microwave until they’re at room temperature or warm and gooey. I recommend heating in 10-second intervals until warmed through; flipping at each interval.