Add the flours, sugar and yeast to the bowl of a stand mixer and stir to combine.
Add warm milk and water and start kneading with the hook attachment until the dough starts pulling away form the bowl (about 12 minutes).
Add salt and softened butter and knead until the dough is soft and elastic (about 15 minutes).
Shape into a ball, place into a bowl, cover and let rise in a warm place until doubled in size (about 1 hour).
After the first rise, deflate dough and divide into 8 equal pieces.
Shape each piece into a ball. Cover and let forse for 15 minutes
Preheat oven to 200°C.Make the topping: combine all the ingredients into a small bowl. The mixture should be thick but spreadable.
Roll the buns into the final shape and place over a lined baking sheet. Spread about 1 tbsp of the mixture on top of each bun.
Pour some hot boiling water into an oven-proof pan and place into the oven.
Bake in the middle rack of the oven for 20 minutes. Transfer to a cooling rack to cool completely