Preheat your oven to 350°F and lightly grease a standard-sized ceramic ramekin with a tiny bit of coconut oil.
In a medium bowl, whisk together the chocolate protein powder, cacao powder, oat flour, baking powder, and sea salt until no lumps remain.
Add the liquid egg whites, Greek yogurt, and melted coconut oil to the dry ingredients, stirring until the batter is smooth and well combined.
Pour approximately two-thirds of the batter into the prepared ramekin.
Place the piece of dark chocolate directly into the center of the batter, then pour the remaining batter over the top to cover it completely.
Bake for 12 to 14 minutes, or until the edges are firm to the touch but the center still has a slight jiggle.
While the cake bakes, place the raspberries, maple syrup, and lemon juice in a small saucepan over medium heat.
Simmer the raspberries for 5 minutes, mashing them with a fork, then press the mixture through a fine-mesh sieve to create a smooth coulis.
Allow the cake to cool for 2 minutes before carefully running a knife around the edge and inverting it onto a plate.
Drizzle the warm raspberry coulis over the lava cake and serve immediately.