In a medium bowl, whisk together the flour, baking powder, nutmeg and salt. In a larger bowl, use an electric mixer to blend together the sugar and butter until light and fluffy. Beat in the egg white and vanilla, scraping the sides of the bowl as needed. Add the flour mixture and the sour cream alternately, beginning and ending with the flour mixture and beating just until blended. Shape the dough into a disk and wrap with plastic wrap. Refrigerate for at least 1 hour.
Preheat the oven to 350℉. Line baking sheets with silicon mats or parchment paper (or spray with nonstick spray).
On a lightly floured surface, roll the dough to a thickness of ¼-inch. Using a 2-inch cookie cutter (I used a heart, but you can use any shape), cut out the cookies re-rolling the scraps to make a total of 28 cookies. Place cookies 1-inch apart on baking sheets.
Bake until the cookies are very lightly browned at the edges, 12 to 14 minutes. Transfer cookies to a rack and cool completely.
PREPARE THE ICING:
In a small bowl, whisk together the powdered sugar, water and vanilla. Add coloring, if using. Use a spoon to drizzle icing over the cookies.