Warm buttermilk in the microwave until slightly warm but not hot. Pour in a Kitchenaide bowl. Sprinkle yeast on top. Let it sit for 5 minutes. Add sugar, salt, vanilla, egg and zest. Attach dough hook and turn on the machine. Add the flour a cup at a time. After the flour is in, add the softened butter a tbsp at a time. Let the dough kneed for a few minutes. Place the dough in an oiled bowl, cover with plastic wrap and place in the refrigerator overnight.Roll out dough into a rectangle. Spread 1/2 of the nutella over the middle 1/3 of the dough.  Fold one side of the dough over the middle 1/3. Spread remaining nutella over the dough that just covered the middle 1/3. Cover with first 1/3 of the dough. Roll it out more. Trim off the 4 sides of the dough. Cut dough in half lengthwise. Cut each half into 6 pieces, so there are 12 pieces total.  Slice each piece into thirds, but not all of the way.  Braid each piece folding one strip over the other.  Roll up the braid and place in a oiled and floured muffin tin.  Repeat with remaining 11 dough pieces.  Let rise for about an hour and bake in a 350 degree oven for about 20 minutes or until browned.  Cool on a wire rack.