Cut the 1/4 cup softened butter into four pieces and add it to the bowl of a stand mixer fitted with the dough hook. Add the flour, sugar, salt, and yeast. Mix on low-medium speed for about 1 minute, just until everything begins to come together. With the mixer running, slowly pour in the cold milk. Once all of the milk has been incorporated, increase the mixer to medium speed.

Knead the dough in the mixer for a full 5 minutes. You can also knead it by hand on a lightly floured surface for about 5 minutes. The finished dough should be soft and slightly tacky, but not overly sticky. If needed, add additional flour just 1 teaspoon at a time. Avoid adding too much flour, which can make the croissants dry.

To check the dough, gently press it with your finger. It should slowly spring back. You can also check for gluten development by stretching a small piece of dough very thin. If you can stretch it until light passes through without the dough immediately tearing, it is ready.

With lightly floured hands, shape the dough into a ball. Place it onto a lightly floured, parchment-lined or silicone-mat-lined baking sheet. Gently flatten the dough, cover it, and refrigerate for 30 minutes.

Remove the dough from the refrigerator and begin pressing it into a rectangular shape with your hands. Using a rolling pin, roll it into a 14 x 10-inch rectangle. Try to keep the sides and corners as straight and even as possible.

Return the rolled dough to the baking sheet and cover well. Refrigerate for at least 4 hours or overnight. The dough can remain refrigerated for up to 24 hours.

Begin preparing the butter about 35 minutes before you’re ready to laminate the dough. Beat the 1 1/2 cups softened butter with the 2 tablespoons flour until completely smooth and combined.

Transfer the butter mixture onto parchment paper or a silicone baking mat and spread it into an even 7 x 10-inch rectangle. Refrigerate for 30 minutes. The butter should be cold and firm but still flexible enough to bend. If it becomes completely hard, leave it at room temperature for a few minutes before continuing.

Remove both the dough and butter from the refrigerator. Place the butter rectangle in the center of the dough. Fold the ends of the dough over the butter so the butter is completely enclosed. Gently press and pinch the seams closed.

Place the dough on a lightly floured work surface with a short side facing you. Roll it into a 10 x 20-inch rectangle. Try to maintain straight sides and even thickness as you roll. Fold the rectangle into thirds, similar to folding a letter: bring one third toward the center and fold the remaining third over it. This completes your first turn.

If the butter or dough feels warm or overly soft at any point, cover the dough and refrigerate it for about 20–30 minutes before continuing.

Position the folded dough so a short side is facing you again. Roll it back into a 10 x 20-inch rectangle. Fold it into thirds again. Place the folded dough onto the baking sheet, cover, and refrigerate for 30 minutes.

Remove the chilled dough and once again roll it into a 10 x 20-inch rectangle. Fold it into thirds one final time. You have now completed all three turns.

Place the laminated dough onto the lined baking sheet and cover well. Refrigerate for at least 4 hours or overnight. It can remain refrigerated for up to 24 hours.

Line two baking sheets with parchment paper or silicone baking mats. Place the chilled dough onto a lightly floured work surface. Roll it into an 8 x 20-inch rectangle.

Cut the rectangle vertically down the center so you have two long strips, each approximately 4 inches wide. Make 3 evenly spaced horizontal cuts to create 8 rectangles measuring approximately 4 x 5 inches each. Cut each of those rectangles lengthwise in half. You should now have 16 pieces measuring approximately 2 x 5 inches each.

Working with one piece at a time, gently stretch or roll the dough until it is approximately 8 inches long. Be gentle so you don’t crush the laminated layers. Place a few small pieces of chocolate in a single layer along one short end of the dough. Roll the dough snugly around the chocolate. Place the croissant seam-side down on the prepared baking sheet.

Repeat with the remaining dough, placing 8 croissants on each baking sheet.

Loosely cover the croissants. Let them rest at room temperature for 1 hour. Keep them in a normal room-temperature spot rather than somewhere especially warm, since you don’t want the butter layers to melt.

After the first hour, transfer the croissants to the refrigerator for another 1 hour. They can also remain refrigerated for up to 12 hours before baking. The croissants should be cold when they go into the oven.

Preheat the oven to 400°F (204°C).

Whisk together the egg and 2 tablespoons whole milk. Remove the croissants from the refrigerator and lightly brush the tops with the egg wash.

Bake the croissants for approximately 20 minutes, rotating the baking sheets halfway through so they brown evenly. They are ready when the outside is beautifully golden brown. If they begin browning too quickly before they’re finished baking, reduce the oven temperature to 375°F (190°C).

Transfer the freshly baked croissants to a wire cooling rack and allow them to cool for a few minutes before serving. They may settle slightly as they cool. Dust with powdered sugar before serving, if desired.