Heat oven to 350°F (176°C). Line an 8x8-inch pan with parchment paper, leaving an overhang. Lightly spray with cooking spray.
Add 1 cup of chocolate chips to a large bowl. In a microwave-safe bowl, melt the coconut oil. Pour the hot oil over the chocolate chips and stir until melted. If needed, microwave in 10–15 second bursts, stirring between each, until smooth. Let cool slightly.
Whisk the Greek yogurt into the melted chocolate. Add vanilla, salt, baking soda, and sugar; whisk until completely smooth.
Blend oats into a fine powder. Measure out 3/4 cup after blending. Stir into the batter along with the remaining 1/2 cup chocolate chips.
Spread batter evenly into the prepared pan (it will be thick). Bake for 28–32 minutes or until a fork inserted in the center comes out clean. Do not overbake; slightly underbaking gives a fudgier result. Let brownies cool completely in the pan on a wire rack.
Optional Frosting: In a microwave-safe bowl, combine chocolate chips and milk. Microwave in 15-second bursts, stirring between each, until smooth. Stir in Greek yogurt, then whisk in powdered sugar. Briskly whisk until completely smooth. Let sit at room temp for 10 minutes to thicken before spreading over cooled brownies.
Cover the pan and refrigerate for at least 6 hours, or overnight. These brownies are best cold. Once chilled, lift out using the parchment and cut into squares.
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Notes
Note 1: I recommend Nestle’s dark chocolate chips with 53% cacao. Anything darker may taste bitter. Don't substitute cocoa powder for the chips; they’re essential.Note 2: Some coconut oil brands taste stronger than others. I use LouAna, which has a very mild flavor.Note 3: Flavored Greek yogurt (honey or vanilla) makes a big difference in taste. My favorite is Greek Gods® Honey Vanilla.Storage: Store brownies in an airtight container in the fridge for 3–5 days. Freeze individual slices (wrapped well) for up to 2 months.