For the strawberry sauce:
Mix the egg yolk, butter, and condensed milk thoroughly with your hands. Add the cornstarch after sifting it, so that it will not have lumps, and mix thoroughly.
Form the cookies making balls and crushing them with a finger to make a hole in the middle, which will later be filled with strawberry sauce. With the help of a grownup, bake the cooking in the oven at 320°F for 12 to 15 minutes. Allow to cool and set aside.
For the sauce, mince the strawberries and put then in a pan with 2 tablespoons of sugar. Boil for around 5 minutes, until thick.
In each hole of the cookies, add 1 teaspoon of strawberry sauce, and voilà! Enjoy some delicious cookies with your family and friends.