Prepare boiled condensed milk in advance. You can use a store-bought one, or you can cook it yourself at home.
Prepare the dough. Pour the yeast into slightly warmed milk and let it sit for 10 minutes. Break the egg, add salt and sugar. Mix everything thoroughly. Add flour a little at a time, bringing the resulting mass to a dough-like texture. Leave to rise for 20 minutes.
While the dough is resting, prepare the butter. It should be soft, but not liquid. Place the glass in a pan of water so that the water completely covers it. Bring the water to a boil. While the water is boiling, cut the butter into small pieces so that you can cover them with a glass, place the pieces on top of each other. Cover with a hot glass and wait until the glass cools down. The oil will soften and become suitable for work.
Start preparing the puff pastry. Roll out the dough to form a layer 0.5 cm thick. Lubricate it thoroughly with oil. Form square envelopes by folding all edges toward the center. Apply a little pressure on top and run the rolling pin gently, making sure the dough doesn't tear. In this case, the oil will spill out of it. Fold the envelope again - and so on for 2 more approaches. Let the dough rise for 15 minutes.
While the dough is “ripening”, extract the yolk from the egg to grease the croissants with it. Break the shell into 2 halves and pour the contents back and forth until the whites are completely poured into the saucer. Pour the remaining yolk into a small bowl and beat well. For the time being, set the saucer aside.
Wrap the dough into croissants. Divide the dough into small lumps and carefully roll each one out. Cut the dough into triangles. Place a teaspoon of filling at the base of the triangles and roll the croissants from the base to the top.
Start heat treatment of croissants with condensed milk. Spread them all over the baking sheet and leave for 10 minutes to allow them to rise a little. Brush the top side of each with yolk. Place in the oven for 20 minutes at 180 degrees until golden brown. After this, the croissants can be taken out and brought to the table.
Place a teaspoon on a saucer and sift cinnamon through a strainer onto the surface of the saucer. Remove the spoon and you will have an original silhouette for decorating the dessert. Place the croissant on a plate and serve with coffee or cocoa.