Preheat your oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper, making sure the edges overhang slightly so the cake lifts out easily later. Lightly spray the corners of the pan so the parchment sits flat and doesn’t curl while filling.
In a large mixing bowl, whisk together the whole eggs and egg whites until they become slightly frothy and light in texture. This helps the cake rise without becoming dense.
Add the Greek yogurt and your preferred sweetener directly into the egg mixture. Whisk slowly until the mixture becomes creamy and completely smooth with no lumps.
Pour in the melted dark chocolate (if using) while whisking gently. Ensure the chocolate is warm but not hot to prevent curdling the eggs. Mix until fully incorporated and velvety.
In a separate bowl, combine the almond flour, protein powder, cocoa powder, baking powder, baking soda, and a pinch of salt. Whisk well to remove any clumps and evenly distribute the leavening agents.
Gradually fold the dry ingredient mixture into the bowl of wet ingredients. Use a spatula and fold slowly to avoid overmixing, which can make the cake tough. Continue folding until everything is just combined.
Check the batter’s consistency — it should be thick like brownie batter. If it feels too dense or dry, add 1 tablespoon of milk at a time, mixing gently after each addition, until the texture becomes smooth and scoopable.
Transfer the batter into the prepared baking pan. Smooth the top with a spatula, spreading the batter into all corners for even baking.
Place the pan into the preheated oven and bake for 20–24 minutes. Begin checking at the 20-minute mark. Insert a toothpick into the center: it should come out slightly moist but not runny. This is key for maintaining the cake’s fudgy texture.
Remove the pan from the oven and let the cake cool inside the pan for about 10 minutes. Then carefully lift it out using the parchment and transfer it to a cooling rack. Allow it to cool completely — around 15–20 minutes more — so the high-protein structure sets properly.
Once fully cooled, slice the cake into 9 equal squares using a sharp knife. Wipe the knife between cuts if you want perfectly clean edges. Enjoy immediately or store in an airtight container for a fudgy treat anytime.