Combine water, buttermilk, sugar and yeast. Add the rest of the ingredients (except 1 tbsp. butter) and knead to a very smooth dough. Add some more water/flour if the dough is too dry/sticky.
Cover the dough with plastic wrap or a damp kitchen towel and let it rest for 2 hours at room temperature.
Shape the dough into burger buns. Let them rise for another 20 minutes while you preheat the oven to 180 °C.
Brush the buns with the remaining tbsp of soft butter. Sprinkle with sesame seeds and bake for about 15-20 minutes.