Preheat oven to 400 degrees F. Butter and dust an 8-inch ramekin with coca powder, set aside.
In a microwave safe bowl, combine butter and chocolate. Microwave on high, for 1 minute in 30 second intervals until melted. Stir in vanilla.
Fitted with a whisk attachment, whip the egg in the bowl of a stand mixer until foamy, about 1 minute. Gradually add sugar and salt, continue to whip the egg until is very thick and pale yellow.
Fold the egg mixture into the chocolate mixture, than sift the flour over top. Incorporate together until no streaks remain.
Pour half of batter into ramekin. Place the square of caramel on top. Cover with remaining batter and bake cake for about 10 minutes. Serve immediately.