Preheat the oven to 170°C (338°F).
Melt dark chocolate and unsalted butter using a double boiler.
In a bowl, combine eggs, erythritol, and vanilla extract and mix well.
Add protein powder and cocoa powder and mix together.
Add the melted chocolate and butter; mix evenly.
Pour the batter into a pan lined with parchment paper.
Tap the pan 2-3 times lightly on the counter to remove air bubbles.
Bake in the preheated oven for 20-25 minutes.
Let it cool completely, then refrigerate for at least 2 hours until set.