Whisk warm milk, yeast, and sugar in a mixer bowl. Let sit 5 minutes until foamy.
Add butter, 2 cups flour, and salt. Mix until combined, then add remaining flour and mix until a soft dough forms.
Knead for 5 minutes until smooth and elastic.
Place dough in a greased bowl, cover, and let rise for about 2 hours until nearly doubled.
Punch down dough and shape into a 10×14-inch rectangle. Chill for 20 minutes.
Beat butter with flour until smooth. Spread butter over the center third of the dough.
Fold dough like a letter, roll out, fold again, and chill for 20 minutes.
Continue laminating as instructed until layers are formed.
Roll dough thin, sprinkle with cinnamon sugar, add optional filling, roll tightly, slice, and place into a greased muffin pan.
Bake until golden and flaky. Cool slightly and enjoy warm.