Preheat your oven to 375°F and lightly grease a 6-ounce ramekin with the coconut oil.
In a medium bowl, whisk together the chocolate protein powder, raw cacao powder, oat flour, and baking powder until well combined.
Stir in the egg whites and nonfat Greek yogurt, mixing until the batter is smooth and no dry clumps remain.
Pour half of the chocolate batter into the prepared ramekin, place the dark chocolate piece in the center, and top with the remaining batter.
Bake for 12 to 14 minutes until the edges are firm but the center still has a slight jiggle when shaken.
While the cake bakes, combine raspberries, lemon juice, and maple syrup in a small saucepan over medium-low heat.
Simmer the raspberry mixture for 5 minutes, mashing the berries with a fork until the sauce is thick and glossy.
Remove the cake from the oven, let it rest for 2 minutes, then carefully invert it onto a plate and top with the warm raspberry coulis.