PREP: Preheat the oven to 350 degrees F (176 degrees C). Line an 8x8-inch pan with parchment paper, leaving an overhang for easy removal. Lightly spritz with nonstick cooking spray.
MELT CHOCOLATE: In a large bowl place 1 cup (175g) of chocolate chips and set aside. Add the solid coconut oil to a separate, microwave-safe bowl. Microwave the coconut oil and then measure the coconut oil in its melted, hot state to get 1/4 cup + 2 tablespoons. Pour the hot melted coconut oil over the dark chocolate and stir the chocolate until it is completely melted. If needed, microwave the chocolate and coconut oil in 10-15 second bursts, stirring in between each burst for 20 seconds until completely melted and smooth. Let cool slightly.
REMAINING INGREDIENTS: Add in the Greek yogurt; stir to combine. Stir in the vanilla, salt, baking soda, and coconut sugar (or brown sugar).
BLEND OATS: Add old-fashioned oats to a blender or food processor. Blend or process until the oats are fine and resemble flour. Measure the oat flour after blending and not before, to get a level 3/4 cup. Add blended oats and remaining 1/2 cup (91g) chocolate chips to brownie batter. Mix to combine.
BAKE: Spread the mixture (it is very thick) into the prepared pan in an even layer. Bake in the preheated oven for 28-32 minutes or until a fork comes out clean when inserted into the center. Do not overbake. Slightly underbaking is better and will yield a superior taste and texture (more fudge-like). Remove from the oven and allow the pan of brownies to cool completely at room temperature on a wire cooling rack. If desired, frost the cooled brownies (see next step).
OPTIONAL FROSTING: In a microwave-safe bowl, combine the 1/2 cup chocolate chips and 2 tablespoons milk. Microwave in bursts of 15 seconds, stirring for 15 seconds in between each burst until the chocolate is completely melted. Stir in the Greek yogurt until fully incorporated and then whisk in the powdered sugar. Frosting may seem thin -- don't worry; it thickens as it cools. Let stand at room temperature for 10 minutes and then spread frosting evenly over the brownies.
CHILL: Cover the frosted (or unfrosted) brownies and place them in the fridge to fully chill. These brownies are best cold (we like them best after being chilled 6-8 hours). Once chilled through, remove the brownies using the overhang of the parchment paper. Cut into small pieces and enjoy!
STORAGE: Store leftover brownies in an airtight container in the fridge for 3-5 days. Freeze individually wrapped brownies for up to 2 months. The frosting does get a bit watery upon being thawed.