Preheat the oven to 350°F. Line an 8 x 8 baking pan with parchment paper.
Mix the oat flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
In a microwave-safe bowl, add about ¾ cup or 75g chopped chocolate and butter. Microwave on low in 15-second increments, stirring after each one, until melted and smooth.
In another medium bowl, add the eggs and erythritol or sweetener of choice and whisk to combine. Add in the Greek yogurt.
Pour the melted chocolate and whisk until combined.
Add the dry ingredients and shredded apple. Stir until just incorporated.
Pour the batter into the baking pan. Top with remaining chocolate and bake for 25-32 minutes.
Allow the brownie to cool, cut into 16 squares and enjoy!