In a medium bowl, vigorously whisk together the all-purpose flour, baking powder, and kosher salt to ensure the leavening agents are perfectly distributed. Set aside.

In a large ceramic or stainless steel mixing bowl, cream the softened butter and granulated sugar together by hand (or with a mixer) until the mixture becomes highly pale, aerated, and fluffy.

Stream the beaten egg and the whole milk into the butter emulsion, mixing until completely unified and smooth.

Gradually fold the dry flour mixture into the wet ingredients in batches. Mix only until the dough just comes together to prevent overworking the gluten, which leads to tough, shrinking cookies.

Divide the unified dough tightly into two equal halves. Wrap each half securely in heavy plastic wrap.

Refrigerate the wrapped dough for a minimum of 2 hours. This critical thermal resting phase solidifies the butter and relaxes the gluten, ensuring the cookies maintain their sharp, architectural edges during the bake.

Preheat the oven to 375°F. Line heavy metal baking sheets with parchment paper or silicone baking mats.

Generously dust a clean, cool work surface (preferably marble) and a heavy wooden rolling pin with confectioners’ sugar (which prevents sticking without toughening the dough like excess flour does).

Remove one disk of dough from the refrigerator. Roll the dough evenly to a strict, uniform thickness of just under 1-inch, rotating the dough frequently to ensure it does not stick.

Use sharp metal cookie cutters to punch out the desired shapes. Transfer the cutouts to the prepared baking sheets, leaving space between each.

Bake in the preheated oven for 9 to 10 minutes, until the edges are just barely turning a pale golden brown.

Allow the cookies to cool on the baking sheet for 2 minutes before transferring them to a wire rack to cool completely to room temperature.

Once fully cooled and structurally set, decorate the cookies by outlining and flooding them with a smooth icing. Garnish immediately with elegant sprinkles before the icing hardens.