Add cold butter and cut into small pieces. Crush and rub the butter into the dry ingredients with your fingers until the mixture resembles coarse sand. Add sweetened condensed milk and mix until it forms a cookie dough ball. If the dough seems too dry, add 1 or 2 tablespoons of water.
Wrap the dough in plastic film and chill in the refrigerator for 30-45 minutes.
Preheat the oven to 390°F (200°C). Using a rolling pin, roll out the dough between two sheets of parchment paper to a thickness of 3-4 mm.
Cut out the cookies using your favorite cookie cutter, for example, a round-fluted cutter. If the dough gets too soft to handle, place it back in the fridge for a few minutes.
Arrange the cutouts on a baking tray covered with parchment. Optionally, add a few flaked or slivered almonds to create a flower pattern. Then, just press them down slightly. For the second batch, re-roll the leftover dough, chill it if necessary, and cut out cookies until the dough is used.
Bake cookies in the preheated oven for 8 to 10 minutes, until the edges are lightly golden brown. Don't overbake your cookies: they tend to color quickly.
Remove cookies from the oven and let them cool down on the baking sheet for 5 to 10 minutes, then transfer them to a cooling rack. Dust with icing sugar.
Notes
Measure flour correctly with a digital scale or the scoop-and-level method.
Don't skip the chilling time; it is essential to make these cookies.
Use an adjustable rolling pin to get a perfectly even dough thickness.
Use cookie sheets interchangeably with baking sheets if you own both.
Don't overbake cookies; they tend to burn very quickly. 8-10 minutes is enough for them to be fully baked.