Heat oven to 350 (175C) degrees and brush the bottom and sides of a 9-inch springform pan with melted butter.
Whisk together sugar, flour, chocolate chips, cocoa and salt in a small bowl. Use your fingers or a fork to blend in the butter until the mixture resembles damp sand. Stir in hazelnuts.
Make the cake
Whisk the flour, baking powder, baking soda and salt together.
In a stand mixer, beat butter and sugar until light and fluffy, about 5 minutes. Add the eggs, one at a time, beating well in between.
Lower mixer speed and add buttermilk, followed by half the flour mixture, mixing until flour is incorporated. Mix in the sour cream and the remaining flour and mix briefly to form a thick, creamy batter.
Scrape the batter into pan. Crumble the topping evenly over the batter.
Bake 1 hour, or until center of cake feels softly springy and a toothpick inserted into the center comes out clean.
Cool in the pan 15 minutes before releasing sides. Set on a rack to cool completely.
Dust a little cocoa powder around the edge of the cake before slicing, if you like.