Preheat your oven to 350°F and lightly grease a 6-ounce ceramic ramekin with the coconut oil.
In a medium mixing bowl, whisk together the egg whites and Greek yogurt until smooth and well combined.
Add the chocolate protein powder, cocoa powder, coconut flour, baking powder, and sea salt to the wet ingredients, stirring until a thick batter forms.
Pour the batter into the prepared ramekin, smoothing the top with a spatula.
Press the piece of dark chocolate into the center of the batter until it is completely submerged and covered.
Bake for 12 to 14 minutes, or until the edges of the cake are firm to the touch but the center still has a slight jiggle.
While the cake is baking, combine the raspberries, maple syrup, and lemon juice in a small saucepan over medium-low heat.
Simmer the raspberry mixture for 5 minutes, mashing the berries with a fork until they break down into a thick, syrupy sauce.
Remove the cake from the oven and let it rest for 2 minutes before carefully inverting it onto a serving plate.
Drizzle the warm raspberry coulis over the top of the cake and serve immediately while the center is still molten.