Chocolate sticks called “batons” are made especially for rolling easily into pain au chocolat. Here, two batons are spiralled into the dough so you get rich pockets of chocolate in each bite of flaky croissant. You can order batons online, but regular chocolate bars, cut crosswise into thin sticks, work just as well. Either way, use a good-quality chocolate.

You'll need a batch of croissant dough, from this series' lead recipe, rolled out to a 38-by-40-cm slab, and chilled. (For the dough, follow the croissants recipe through to step 16.)

1 Arrange racks in the upper third and lower thirds of the oven. Bring a cast-iron frying pan of water to a simmer over medium-high heat. Transfer the pan to the floor of the oven and close the door. (The steam released inside the oven will create an ideal proofing environment for the pains au chocolat.) Line two rimmed baking sheets with parchment paper and set aside.

2 Remove the slab of dough from the refrigerator and let sit for 5 minutes at room temperature. Unwrap (save the plastic for proving) and place on a very lightly floured surface. If the dough has shrunk during chilling, roll it out again to a slab that's 40cm long and 38cm wide. Dust off any excess flour with a pastry brush.

3 Use a wheel cutter to trim 1cm of dough from all four sides, straightening and squaring them off, creating a slab that's 38cm long by 35cm wide. Using a ruler, cut the slab lengthwise into 5 equal strips each measuring 7cm wide. Cut each strip in half crosswise, creating 10 rectangles.

4 Working one rectangle at a time, place a stick of chocolate along one of the shorter sides, leaving about a 2.5cm border. Fold the pastry over the chocolate until it's wrapped around one time, then tuck another bar of chocolate into the fold. Wrap the pastry around the second bar of chocolate and continue to roll until you have a snug spiral. Transfer the pain au chocolat to a prepared baking sheet, resting it on the seam. Repeat with the remaining dough and chocolate, dividing between the baking sheets and spacing evenly. Very loosely cover with plastic wrap so the pastries have some room to expand.

5 Open the oven and stick your hand inside – it should be humid but not hot, as the water in the frying pan will have cooled. You want the pains au chocolat to proof at 21 to 23 degrees. (Any hotter and the butter will melt, leading to a denser pastry.) Gently place the baking sheets inside the oven and let the pastries proof until they have doubled in size, are extremely puffy and jiggle delicately on the baking sheet, 2 to 2½ hours.

6 Remove the baking sheets from the oven and carefully uncover them. Transfer to the refrigerator and chill for 20 minutes while you heat the oven. Remove the frying pan from the oven and heat to 190 degrees.

7 In a small bowl, stir the yolk and double cream until streak-free. Remove the baking sheets from the refrigerator and use a pastry brush to gently brush each pain au chocolat with the yolk mixture. Transfer the sheets to the oven and bake for 20 minutes. Rotate the baking sheets and switch racks, and continue to bake until the pains au chocolat are deeply browned, another 10 to 15 minutes. Remove from the oven and let cool completely on the baking sheets.

Pains au chocolat are best within an hour or two of baking. After that, revive them by warming in a 170-degree oven for 5 to 8 minutes. Keep wrapped airtight at room temperature. – New York Times

This recipe is from a series
Introduction: How to make stunning croissants at home
Recipe: Croissants
Recipe: Ham and cheese croissants
Recipe: Almond croissants