01 - In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
02 - In a large bowl, beat the softened butter and sugar until light and fluffy, approximately 2 to 3 minutes.
03 - Add the egg and vanilla extract; beat until combined.
04 - Gradually add the dry ingredients; mix until a smooth dough forms.
05 - Divide the dough in half, shape into discs, wrap in plastic wrap, and chill for at least 1 hour.
06 - Preheat oven to 350 degrees Fahrenheit. Line baking sheets with parchment paper.
07 - On a lightly floured surface, roll out dough to 1/4-inch thickness. Cut out heart shapes with a cookie cutter. Place cookies 1 inch apart on prepared sheets.
08 - Bake for 8 to 10 minutes, or until edges are just beginning to turn golden. Cool completely on a wire rack.
09 - In a large bowl, combine powdered sugar and meringue powder. Add 4 tablespoons warm water.
10 - Beat with an electric mixer on low, then high, until stiff peaks form, approximately 4 to 5 minutes.
11 - Adjust consistency with more water, a few drops at a time, for flooding or piping as desired.
12 - Divide icing into bowls and tint with gel food coloring if using.
13 - Decorate cooled cookies with the royal icing. Allow to set completely before stacking or serving.