Make the Cookie Dough
Beat softened butter and sugar in a stand mixer on medium speed until light and fluffy, about 2–3 minutes.
Add egg and vanilla extract; mix until just combined.
In a separate bowl, whisk flour, baking powder, and salt. Add to the butter mixture on low speed; mix until a smooth dough forms. Do not overmix.
Divide dough into two discs; wrap tightly in plastic wrap. Refrigerate at least 1 hour. Do not skip this step — chilled dough keeps flower shapes clean and sharp.
Cut and Bake
Preheat oven to 375°F (190°C). Line baking sheets with parchment paper.
On a lightly floured surface, roll chilled dough to ¼-inch thickness.
Press flower-shaped cookie cutters firmly into the dough for clean cuts. Place cookies 1 inch apart on prepared baking sheet.
Bake 10–12 minutes until edges are just lightly golden. Centers should look set but pale.
Cool on the baking sheet 3–5 minutes, then transfer to a wire rack. Cool completely before decorating — never ice a warm cookie.
Make the Royal Icing
In a stand mixer bowl, combine meringue powder, cream of tartar, and 6 tbsp warm water. Beat on medium until foamy.
Add sifted powdered sugar and vanilla; beat on high 5–7 minutes until glossy, stiff peaks form.
Divide icing into bowls — one for each color plus a white bowl. Tint each with a tiny speck of gel paste color using a toothpick; stir until fully blended. Adjust color gradually — a little goes a very long way.
Outline icing: Keep it thick (toothpaste consistency) — holds its shape when piped. Fill piping bags fitted with a size 1–2 round tip.
Flood icing: Add warm water a few drops at a time to thin the icing until it flows and settles flat within 10–15 seconds (shampoo consistency). Fill squeeze bottles.
Decorate the Cookies
Pipe a clean border around each cookie using outline-consistency icing. Let sit 2–3 minutes to set.
Fill the inside of the border with flood icing using a squeeze bottle. Use a toothpick to guide icing into edges and pop any air bubbles.
Wet-on-wet polka dots: While the flood icing is still wet, pipe small dots of a contrasting color (or white) flood icing on top. Drag a toothpick through the center of each dot for a teardrop effect.
Add nonpareil sprinkles or pearl centers immediately to the flower center while icing is wet so they adhere.
Let cookies dry fully at room temperature — at least 6 hours, or overnight for best results — before stacking or packaging.