Coat the inside of four coffee mugs with non-stick cooking spray, set aside. With a hand blender on low speed, mix together the eggs, butter, milk, yogurt and cake mix, until just combined. Scrape the sides of the mixing bowl, then beat on high speed for 1 minute.
Place 1/4 cup of cake batter into the bottom of each mug, followed by a tablespoon of chocolate hazelnut spread. Repeat so each mug has three layers of batter and chocolate hazelnut spread.
Place the semi sweet chocolate in a microwave safe bowl and heat in 30 second increments until melted and smooth.
Heat each mug in the microwave for two minutes. Top with desired garnishes then using a spoon, drizzle the melted chocolate over the top of each mug.