If using compressed yeast, in the bowl of a heavy-duty stand mixer, combine the crumbled yeast and sugar. Let stand until the yeast gives off some moisture, about 3 minutes, then whisk well to dissolve the yeast. Add the sour cream, lemon juice, vanilla, and egg yolks and whisk well to combine. Whisk in the milk until smooth.
If using active dry yeast, in a small bowl, sprinkle the yeast over 1/4 cup lukewarm, 105° to 115°F (41° to 46°C), milk. Let stand until the yeast softens, about 5 minutes, then stir to dissolve. Pour into the bowl of a heavy-duty stand mixer. Add the sugar, sour cream, lemon juice, vanilla, and egg yolks and whisk well to combine. Whisk in the remaining 1/2 cup cold milk until smooth. Attach the bowl to the mixer and fit the mixer with the paddle attachment.
On low speed, gradually add half the flour and then the salt. Mix until a sticky, batter-like dough that clings to the sides of the bowl forms. Increase the speed to medium. Beat in half of the well-softened butter, 1 tablespoon at a time, letting each addition be absorbed before adding another. Replace the paddle attachment with the dough hook. Mix in half the remaining flour, then the remaining butter, and then the remaining flour, letting each addition be absorbed before adding the next. Knead on medium-low speed until the dough is smooth and soft, about 5 minutes.
Transfer the dough to a lightly floured work surface. Knead briefly to check the dough’s texture: It should feel slightly sticky but not stick to the work surface. Knead in more flour only if needed. Generously butter a large bowl. Shape the dough into a taut ball. Place it in the bowl, turn to coat it with butter, and then turn it smooth side up. Cover tightly with plastic wrap. Let stand in a warm place until the dough doubles in volume, about 45 minutes.
Working carefully to preserve the dough’s light and puffy texture, turn the dough out onto a lightly floured work surface. (Do not knead the dough.) Cut the dough in half and shape each piece into a 1-inch-thick rectangle. Cover with a kitchen towel and let rest for 15 minutes before using. If not using immediately, do not let the dough rest. Instead, immediately wrap each half tightly in plastic wrap and freeze for up to 2 weeks. To use the frozen dough, transfer to the refrigerator and let thaw for at least 8 hours, or up to overnight.