In a large mixing bowl, dissolve the active dry yeast in warm milk. Let it sit for about 5 minutes until it becomes frothy.
Add the flour, sugar, and salt to the yeast mixture. Mix well until a shaggy dough forms.
Turn the dough onto a floured surface and knead for about 5-7 minutes until it becomes smooth and elastic.
Place the dough in a greased bowl, cover it with a clean kitchen towel, and let it rise in a warm place for about 1 hour or until it doubles in size.
Prepare the butter block by flattening softened butter between two sheets of parchment paper into a rectangle about 1/2 inch thick. Chill it in the refrigerator.
Once the dough has risen, punch it down and roll it out into a large rectangle. Place the chilled butter block in the center, folding the dough over the butter.
Roll the dough out into a long rectangle, then fold it into thirds like a letter. Repeat this rolling and folding process two more times, chilling the dough for 30 minutes between each fold.
After the final fold, roll the dough into a large rectangle again. Cut it into triangles, placing a dollop of almond paste at the base, and roll it up tightly.
Place the shaped croissants on a lined baking sheet. Cover them with a towel and let them rise for another hour.
Preheat your oven to 400°F (200°C). Beat the egg and brush it over the tops of the croissants. Bake for 15-20 minutes until golden brown and flaky.
Allow the croissants to cool slightly on a wire rack. Dust with powdered sugar before serving.
