Prep your ramekins: Lightly grease two 6-ounce ramekins or one 8-ounce ramekin with butter or oil spray.
Preheat the oven to 400°F (200°C). If using an air fryer, preheat to 350°F (175°C).
Mix dry ingredients: In a small bowl, whisk the protein powder, cocoa powder, oat flour, baking powder, salt, and espresso powder (if using) until no lumps remain.
Combine wet ingredients: In another bowl, whisk the egg, milk, Greek yogurt, sweetener, and vanilla until smooth.
Bring it together: Fold the dry ingredients into the wet just until combined. The batter should be thick but scoopable.
If it feels dry, add 1–2 teaspoons more milk.
Fill and add the “lava”: Spoon half the batter into the ramekin(s). Place the chopped dark chocolate in the center. Top with the remaining batter, making sure the chocolate is fully covered.
Bake: Bake 8–10 minutes (ramekins vary).
The edges should look set, but the center should have a slight jiggle. In an air fryer, start checking at 6–7 minutes.
Rest briefly: Let the cake sit for 1 minute to finish setting while staying molten inside.
Serve: Run a knife around the edge, invert onto a plate, and lift off the ramekin. Or eat straight from the dish.
Add a scoop of Greek yogurt, a sprinkle of cocoa, or fresh berries if you like.