01 - In a medium bowl, whisk together flour, baking powder, and salt until thoroughly combined. Set aside for later use.
02 - In a large bowl, beat softened butter and granulated sugar with an electric mixer until light and fluffy, approximately 2-3 minutes.
03 - Add the egg and vanilla extract to the butter mixture. Beat until fully incorporated and smooth.
04 - Gradually mix in the dry ingredients on low speed until a soft dough forms. Avoid overmixing to maintain tender texture.
05 - Divide dough in half, shape into flat discs, wrap tightly in plastic wrap, and refrigerate for at least 1 hour until firm.
06 - Preheat oven to 350°F. Line 2 baking sheets with parchment paper and set aside.
07 - On a lightly floured surface, roll out chilled dough to 1/4-inch thickness. Cut out heart shapes using cookie cutters.
08 - Place cut cookies 1 inch apart on prepared baking sheets to allow for slight spreading during baking.
09 - Bake for 8-10 minutes until edges are just beginning to turn golden. Remove from oven and cool completely on wire racks.
10 - Beat powdered sugar, meringue powder, and 3 tablespoons water until stiff glossy peaks form. Adjust consistency with additional water, adding drops at a time for piping or flooding.
11 - Tint icing with gel food coloring as desired. Mix thoroughly until uniform color is achieved.
12 - Decorate completely cooled cookies with royal icing using piping bags or small zip-top bags. Allow icing to set uncovered at room temperature for several hours before serving or packaging.