In a stand mixer combine the starter, butter, water, flour, milk, salt, and sugar
With the paddle attachment, mix for 3 minutes
Replace with hook attachment and mix for 10-15 mins or do the windowpane test (when you stretch the dough, it doesn't break and becomes see thru)
Cover the dough and let it rise in the oven with the light on for around 4 hours. Do a couple of stretches and folds every 2 hours.
On a lightly floured surface, turn the dough and shape into 6 balls (each 120g approx)
Place the balls onto a lightly greased parchment paper and let them rise for another 4hrs, i let them rise for 2hrs and then left them overnight in the fridge
Preheat the oven to 190C, remove the buns from fridge. Brush with the egg yolk mix and add your toppings if any
Bake them for 20-25 mins, until they're golden brown and if you have a thermometer it should read 88C from the middle of the bun
You can store them in a tupperware or freeze them