Warm 1/2c milk and add in sugar and yeast and set aside until yeast looks foamy and bubbly.
While yeast is proofing, in a large bowl or stand mixer, add flour, butter (3T), salt and with dough hook start mixing to incorporate butter. If using instant yeast, you would add that now.
Pour in proofed yeast mixture making sure to scrape in all of the sugar. Incorporate into the flour and then slowly add in the cold milk until dough forms. Knead on medium for 5-7 min until a soft ball forms. If the dough is too sticky then knead for a few more minutes after adding in 2T of flour. Dough should pull away from the bowl in stand mixer or feel soft if kneading by hand. This is a soft dough but you will be rolling it out so it needs to have structure. Once dough is formed, cover with plastic wrap and place in fridge while you make butter layer.
Place 3 sticks of butter in a bowl (stand mixer or you can mix by hand) and 3T of flour. With paddle attachment or with spoon, mix butter and flour until combined BUT do not cream and introduce air into the mixer. Once combined, spoon it onto a piece of parchment or silicone baking mat. Spread with a spatula to create an 8x8in square. You need to use a ruler or measuring tape throughout the process. Cover with plastic and place in the fridge.
Take dough out of fridge and roll into a 12x12in square. Place on a piece of parchment paper or baking mat. Cover with plastic wrap and place in fridge.