Preheat your oven to 350°F and line an 8x8 inch baking dish with parchment paper, allowing the edges to hang over for easy removal later. This prevents sticking and makes cleanup simple.

In a medium bowl, whisk together the protein powder and cacao powder until completely combined and free of lumps. Add the 1 cup of Greek yogurt and vanilla extract, then stir until well incorporated. The mixture will be thick at this point—this is normal and shows the protein and cacao are fully hydrated.

Add the 1/4 cup water a little at a time, stirring gently until you reach a thick brownie batter consistency—it should be spreadable but not runny. I like to use only 1/4 cup water rather than the original 1/2 cup because it creates a denser, more fudgy texture that's more satisfying. Pour the batter into your prepared baking dish and spread it evenly into the corners using a spatula.

Place the baking dish in the preheated 350°F oven and bake for 30 to 40 minutes, until a toothpick inserted in the center comes out with just a few moist crumbs (not wet batter). The brownies will continue to set as they cool, so don't overbake—slightly underbaked is better than dry.

Remove the brownies from the oven and let them cool completely at room temperature, about 30 minutes. Once cooled, refrigerate overnight—this resting period allows the texture to become fudgy and helps the flavors develop. The cold also makes them easier to cut cleanly.

In a small bowl, mix together Greek yogurt with sifted cacao powder until smooth and spreadable, adjusting the cacao powder amount to reach your desired thickness and chocolate intensity. Spread the frosting evenly over the chilled brownies. For the cleanest cuts and presentation, use a warm, damp knife wiped between each slice—this prevents the frosting from dragging.