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.Take out dough from the refrigerator. Roll out dough into a rectangle. Mix ingredients for filling. Spread it over the dough. Roll dough up lengthwise. Cut into 12 equal portions. Place each roll in a parchment lined 9x13 baking pan. Let them rise for an hour. Bake in a 350 degree oven for about 20 minutes or until browned. Sprinkle with powdered sugar.