Whisk flour, baking powder, and salt in a medium bowl. Set aside.

Beat butter and sugar in a large bowl on medium-high speed for 3 to 4 minutes until light and fluffy.

Add egg and vanilla extract. Beat until combined. Scrape down the bowl sides.

Add flour mixture. Mix on low speed just until dough comes together. Do not overmix.

Divide dough in half. Shape each half into a flat disk. Wrap tightly in plastic wrap.

Refrigerate for at least 2 hours or up to 3 days.

Heat oven to 350°F. Line baking sheets with parchment paper.

Roll one disk of dough on a lightly floured surface to 1/4 inch thickness. Keep the other disk refrigerated.

Cut out heart shapes with cookie cutters. Place cookies 1 inch apart on prepared baking sheets.

Bake for 10 to 12 minutes until edges just begin to turn golden.

Cool on baking sheet for 5 minutes. Transfer to wire rack to cool completely.

Make royal icing: Wipe mixer bowl and whisk with vinegar to remove grease.

Sift confectioners' sugar through a fine mesh sieve into a large bowl.

Whisk meringue powder and warm water in mixer bowl by hand until foamy.

Add sifted sugar and vanilla extract to foamy water.

Beat on low speed to combine. Increase to medium-high speed. Beat for 5 to 7 minutes until stiff, glossy peaks form.

Divide icing into bowls. Add gel food coloring. Stir gently.

Thin icing with water drops for flood consistency. Use 15-second rule to test (ribbons should disappear in 15 seconds).

Decorate cooled cookies using outline and flood method. Keep icing covered with damp paper towel between uses.

Let decorated cookies dry for 6 to 8 hours before serving or packaging.