• To a bowl of a stand mixer with a dough hook attachment, add the dry ingredients, sourdough starter, milk, egg, and vanilla.
• Mix on low speed (1 or 2) for about 5 or so minutes or until it becomes smooth and glossy. You can also knead this by hand. Mix and knead to make a shaggy, sticky dough then continue to knead for about 5-10 minutes until smooth and glossy.
• Shape into a ball and place in a greased large bowl. Cover with plastic wrap, foil, or a beeswax wrap and place at room temperature (a warm place) for about 4 hours.
• Transfer the bowl to the refrigerator to continue fermenting for 8 hours, or up to 3 days.

Laminating the dough

• Place the sticks of butter on a large piece of parchment paper. Sprinkle it with a little flour. Mark a 6″ by 8″ rectangle onto the parchment paper.
• Fold the parchment paper around the butter with the folds on that 6" by 8" rectangle you made, so that the butter will fill the entire envelope once you roll it out (also called a butter packet). It doesn’t need to be perfectly precise.
• With a rolling pin, tap out the butter lightly to start to spread the butter, then roll smooth.
• Chill the butter in the refrigerator for 10 minutes. You want the butter to stay nice and cold to create the layers of butter. If the butter gets too warm, it will just melt into the dough.
• Remove the pastry dough from the refrigerator and place on a lightly floured surface.
• Roll it out into a 16″ by 8″ inch rectangle.
• Place the butter rectangle in the middle of the dough. Fold the edges over, so they meet in the middle and completely encase the butter. Pinch the dough together down the middle seam and sides closed, so none of the butter can escape.

First fold

• On a lightly floured work surface, roll the dough, with the butter sealed inside, into a 16″ by 8″ inch rectangle.
• Fold each side into the center; then fold one side over the other to make a rectangle with four layers of folded dough.
• Wrap the dough in plastic wrap and place in the refrigerator for 30 minutes.

Second fold

• Roll the dough out again into a 16″ by 8″ inch rectangle.
• Fold each side into the center; then fold one side over the other to make a rectangle with 4 layers of dough.
• Wrap the dough in plastic wrap and put it back in the refrigerator for 2 hours or up to 12 hours.
• Use in any recipe calling for puff pastry.