1 /2 cup milk
2 1/2 teaspoons active dry yeast
3 cups unbleached all-purpose flour
8 tablespoons cold unsalted butter, cut into 12 pieces
1/4 cup sugar
1 teaspoon salt
2 large eggs
2 large egg yolks
1 tsp poppy seeds
1 additional large egg, beaten with a pinch of salt
Heat the milk in a small saucepan or in the microwave until it is just lukewarm, about 110 degrees F. Pour the milk into a small bowl and whisk in the yeast. Use a spoon to stir in 1 cup of the flour. Cover the bowl with plastic wrap and set aside until it is bubbly and slightly risen, about 20 minutes.
In the bowl of a food processor fitted with the metal blade (or the dough blade, if you have one), combine the butter, sugar, salt, eggs and egg yolks. Pulse until the butter is finely chopped and equally distributed throughout the mixture, which will appear curdled and separated (but won’t worry–it will come back together when flour is added).
Scrape the yeast mixture into the food processor and pulse 6 times to mix. Add 1 cup of the remaining flour and pulse until the mixture is smooth. Scrape down the bowl. Add the rest of the flour and pulse again until well-mixed. Let the dough rest in the food processor for 10 minutes.
After the rest, start the processor and let it run continuously for 10 seconds. Turn the dough out onto a floured work surface. With the help of a bench scraper, knead the dough 5 or 6 times, or until it is slightly more elastic.
With the bench scraper or a knife, divide the dough into 3 equal pieces. Roll each each piece into a thick, even rope about 12 inches long. If the dough is too sticky to work with, refrigerate it for 20 minutes and start again.
Line a baking sheet with parchment paper. Place the three ropes lengthwise on the sheet, with about 1/2 inch in between them. Starting in the middle of the loaf, braid the ropes together towards you, pinching the ends together gently and tucking them under the loaf. Rotate the pan and repeat, braiding the rest of the loaf.
Cover the braid with a towel or buttered plastic wrap and let rise until doubled in bulk, 1 to 2 hours, depending on how warm the room is.
When you’re ready to bake, position a rack to the center of the oven and preheat it to 350 degrees.
Just before baking, brush the braid lightly all over with the egg wash, being careful not to let it puddle in the creases of the braid, spread some poppy seeds on the top. Bake until the bread is well risen and beautifully golden and glossy, about 35 to 40 minutes. Slide the parchment onto a wire rack to let the bread cool before slicing.