In the bowl of a stand mixer, whisk together warm milk, egg, egg yolk, instant yeast, and sugar.
Attach dough hook to stand mixer. Add flour and salt to mixing bowl. Beat until dough starts to form. With the mixing running on low speed, add softened butter one tablespoon at a time. Once all the butter has been added, increase to medium speed and knead until dough is smooth and no longer sticks to the sides of the bowl, about 3-5 minutes.
Transfer dough to a large greased bowl. Cover with plastic wrap and allow to rest at room temperature for about 2 hours until doubled in size. Meanwhile, prepare butter block.
Punch down risen dough. Turn out dough onto a lightly floured work surface. Use a small rolling pin, or the palm of your hands, to gently shape dough to a rectangle about 10x8-inches. Place on a parchment lined baking sheet. Tightly cover dough with plastic wrap. Let chill in the fridge overnight.