Preheat the oven to 400 degrees F. Line a 12-tin muffin pan with muffin liners.
In a medium bowl, combine the flour, cocoa powder, salt, baking powder, and baking soda. Stir until fully combined. Set aside.
In a large mixing bowl, add the avocado. Using a fork, mash completely mash the avocado until no more lumps are visible. Add in the eggs, milk, vanilla extract, oil, and maple syrup. Beat together using a hand mixer until fully combined.
Pour the dry ingredients into the wet. Stir together until just combined. Fold in the chocolate chips.
Divide the batter into the muffin tin.
To make the hazelnut streusel, add the hazelnuts, flour, brown sugar, cinnamon, and salt. Mix together. Add in the cold butter and cut into the hazelnut mixture using a pastry cutter or fork until it resembles coarse sand. Evenly top each unbaked muffin with the streusel.
Bake at 400 degrees for 5 minutes. Reduce to 350 degree and bake for an additional 13-16 minutes or until batter is set.