MAKES ABOUT 60
1 1/4 cups milk
12 tbsp. unsalted butter, plus more for greasing
1/3 cup plus 1 tsp. sugar
1 tsp. kosher salt
2 1/4-oz. packages active dry yeast
5 cups flour
1 tbsp. canola oil
1 lb. double-smoked bacon, cut into 1/8″ cubes
1 small yellow onion, minced
Kosher salt and freshly ground black pepper, to taste
1 tbsp. heavy cream
1 egg yolk
1 egg white, lightly beaten
Make the dough: Heat milk, butter, 1/3 cup sugar, and salt in a 2-qt. saucepan over medium heat until sugar dissolves; set aside. Whisk together remaining sugar, yeast, and 1/4 cup water heated to 115° in the bowl of a stand mixer fitted with a dough hook; let sit until foamy, about 10 minutes. Whisk in milk mixture, add flour, and mix on low speed until dough forms. Increase speed to medium-high and knead until smooth, about 8 minutes. Cover bowl with plastic wrap; let sit until doubled in size, about 1 1/2 hours.
Meanwhile, make the filling: Heat oil in a 4-qt. saucepan over medium heat, and add bacon; cook, stirring, until fat renders, about 6 minutes. Add onion and cook, stirring, until onion is lightly caramelized but bacon is not crisp, about 6 minutes. Remove from heat; season with salt and pepper. Let cool completely.
Heat oven to 400°. Whisk together cream and egg yolk in a small bowl; set egg wash aside. Transfer dough to a floured work surface and cut in half. Working with one half at a time, roll dough until 1/4″ thick. Using a 2 1/2″ round cutter, cut out dough rounds. Place 1 heaping tsp. bacon filling in center of each round, and, using your fingers, moisten edges of round with egg white; fold over to enclose filling, and pinch edges together to seal. Transfer turnovers, seam side down, to parchment paper—lined baking sheets, spaced 3″ apart. Using a pastry brush, brush wash over each bun; bake until golden brown, 12–15 minutes.