Make the dough (détrempe): In the bowl of a stand mixer, add the flour, sugar, salt, milk, sourdough starter, and optional active dry yeast. (Note: adding a tiny percentage of active dry yeast can help achieve a proper fermentation process without taking away any of the benefits of sourdough! If you do not have a sourdough starter but would still like to make these croissants with yeast, see the notes below). Using the dough hook attachment, mix on low speed for 1 minute. Add the butter, then continue mixing—scraping down the sides as needed—until the dough is smooth, about 3–4 minutes.
600 g (4 ¾ cups) all-purpose flour, 65 g (⅓ cup) granulated sugar, 12 g (2 teaspoons) salt, 60 g (¼ cup) active sourdough starter 100% hydration*, 205 g (1 cup) water, room temperature , 112 g (½ cup) whole milk, room temperature , 56 g (¼ cup) unsalted butter, cut into ½-inch pieces, chilled , ⅛ - ¼ teaspoon almond extract, 1.5 g (½ teaspoon) active dry yeast (0.2%)