Preheat oven to 175°C (350°F) and coat a 6-ounce ramekin with non-stick spray.
In a medium bowl, whisk together the whey protein powder, oat flour, cocoa powder, baking powder, and erythritol.
Stir in the Greek yogurt, egg white, red food coloring, and almond milk until a smooth batter forms.
Pour two-thirds of the batter into the prepared ramekin.
Place the dark chocolate piece into the center of the batter.
Cover the chocolate with the remaining batter, ensuring it is fully submerged.
Bake for 12 minutes until the outer edges are set but the center remains slightly soft.
Allow the cake to rest for 2 minutes before serving directly in the ramekin.