In a large mixing bowl, or the bowl of a stand mixer fitted with the paddle attachment, cream together the cold, cubed butter and granulated sugar on medium speed until light and fluffy, about 3-4 minutes.

Add the vanilla extract and mix until just combined.

In a separate bowl, whisk together the sifted all-purpose flour and salt. Gradually add the dry ingredients to the wet ingredients on low speed, mixing until just combined and a soft dough forms. Be careful not to overmix.

Divide the dough into two equal portions. Flatten each portion into a disc, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes, or up to 2 hours, to allow the butter to firm up.

Preheat oven to 325°F (160°C). Line two baking sheets with parchment paper.

On a lightly floured surface, unwrap one disc of dough. Roll it out to an even thickness of about 1/4 inch.

Using flower-shaped cookie cutters, cut out the cookies. Carefully transfer the cut cookies to the prepared baking sheets, leaving about 1 inch between them. Reroll scraps as needed, but try not to overwork the dough. Repeat with the second dough disc.

Bake for 12-15 minutes, or until the edges are lightly golden and the centers are set. The cookies should still be pale.

Remove from oven and let cool on the baking sheets for 5 minutes before transferring to a wire cooling rack to cool completely.

Once cookies are completely cool, prepare the icing. In a medium bowl, whisk together the sifted powdered sugar, milk (start with 3 tablespoons and add more a tiny bit at a time until desired consistency), and vanilla extract until smooth. The icing should be thick enough to hold its shape but pourable.

Divide the icing into several small bowls (one for each color you plan to use). Add a drop or two of gel food coloring to each bowl and mix well to achieve desired pastel shades (pink, yellow, green, blue).

Carefully spoon or pipe the colored icing onto the cooled flower shortbread cookies. You can flood the entire cookie or create delicate patterns.

Use a toothpick or a fine brush to spread the icing to the edges and create details like flower petals or centers.

Immediately add sprinkles, if using, before the icing sets.

Allow the decorated cookies to set completely at room temperature for at least 1-2 hours, or until the icing is firm to the touch.