Pre-heat oven to 350°, grease and flour a 9 inch tube pan.
In a small bowl mix together cocoa, sugar and hazelnuts (the topping) and set aside.
In a large bowl cream butter, beat in 1 cup of sugar and eggs, one at a time, beat well after each addition.
In a medium bowl sift flour, baking powder, baking soda and salt together.
In a small bowl mix greek yogurt and vanilla together.
Then starting with the flour mixture add to egg mixture then yogurt mixture (ending with yogurt).
Pour half the batter into tube pan, sprinkle half the topping over the batter, and add the remaining batter and cover with the remaining topping. With a knife run it through the batter to form a marble effect. Bake for approximately 1 hour (check after 45 minutes) Enjoy!