Preheat the oven to 350°F. Lightly grease a small cast iron skillet or baking dish.
Melt the dark chocolate chips with the coconut oil until smooth. Set aside to cool slightly.
In a mixing bowl, whisk the eggs, coconut sugar, and vanilla until light and fluffy, then beat in the Greek yogurt and peanut butter until smooth.
Slowly whisk in the melted chocolate mixture until fully combined.
Fold in the almond flour, cocoa powder, protein powder, salt, and espresso powder until the batter is glossy and smooth. Fold in optional mini chocolate chips if using.
Pour the batter into the prepared skillet and smooth the top evenly.
Bake for 25 minutes only. The center should still look slightly underbaked when you pull it, and that is exactly what you want.
Let the skillet rest for 10 to 15 minutes before serving. The carryover heat gently finishes the center while it stays fudgy. Baking all the way to a clean toothpick dries out the protein, so underbake, then rest.