In a small bowl, mix warm milk, sugar, and yeast. Let sit for 5-10 minutes until foamy.
In a large bowl, mix flour and salt. Add the yeast mixture and knead into a dough. Cover and refrigerate for 30 minutes.
Roll out the dough into a rectangle and place thin slices of cold butter evenly over it. Fold the dough into thirds, then roll it out again. Repeat this process 3 times to create flaky layers.
Wrap the dough in plastic wrap and refrigerate for at least 1 hour or overnight.
Roll the dough into a thin rectangle and cut into small triangles. Roll each triangle from the wide end to the tip to form mini croissants.
Place the croissants on a baking sheet lined with parchment paper and let them rise for 30-45 minutes.
Preheat the oven to 375°F (190°C).
Whisk together egg yolk and milk, then brush the tops of the croissants.
Bake for 15-18 minutes, or until golden brown and flaky.
Let cool slightly before serving warm with butter, jam, or honey!
Notes
For an extra touch, sprinkle the croissants with sesame seeds or a light dusting of sugar before baking.