Prep the pan: Heat the oven to 350°F (175°C). Line an 8x8-inch pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.
Melt the chocolate: In a microwave-safe bowl, melt the dark chocolate in 20–30 second bursts, stirring between each, until smooth.
Set aside to cool slightly.
Mix dry ingredients: In a large bowl, whisk oat flour, protein powder, cocoa powder, baking powder, salt, and optional espresso powder until no lumps remain.
Whisk wet ingredients: In another bowl, whisk Greek yogurt, applesauce, eggs, maple syrup, brown sugar, oil, and vanilla until smooth and glossy.
Add melted chocolate: Stir the melted chocolate into the wet mixture until fully combined. It should look thick and shiny.
Combine: Pour the wet mixture into the dry. Fold with a spatula until just combined.
Do not overmix.
Stir in chips: Fold in most of the mini chocolate chips, reserving a small handful for the top.
Pan and top: Spread the batter evenly in the prepared pan. Sprinkle the remaining chips over the surface. Add flaky salt if you like.
Bake: Bake for 18–24 minutes, until the center is just set and a toothpick comes out with a few moist crumbs.
For extra fudgy brownies, err on the shorter time.
Cool completely: Let the pan cool on a rack for at least 45–60 minutes. Lift out with the parchment and slice into 12 squares with a sharp knife.