Make the Sugar Cookies:

1. In a large mixing bowl, cream butter and sugar together using a hand mixer (or stand mixer) on medium speed until light and fluffy, about 2 minutes. Add egg and vanilla and beat until combined. Turn the mixer to low speed and add baking powder and salt. Gradually add flour and mix until combined.
2. Form the dough into a ball and wrap it tightly in plastic cling wrap. Freeze for 30 minutes or refrigerate until firm (at least 1 hour, up to overnight).
3. Preheat the oven to 350 F. Line a half sheet baking pan with parchment paper or silicone baking mat.
4. Remove the dough from refrigerator and let it sit at room temperature for 5-10 minutes. On a lightly floured surface, use a rolling pin to roll the dough out until it is 1/4-inch thick. Use a 3-inch heart-shaped cookie cutter to cut out heart cookie shapes.
5. Transfer the cookies onto the lined baking sheet, placing them one inch apart. Re-roll any scraps and repeat to cut out more cookies.
6. Bake cookies for 8-10 minutes, until the edges start to turn golden brown. Let the cookies cool for a couple minutes on the baking sheet, then transfer them to a wire cooling rack to cool completely.

Prepare the Royal Icing:

1. In medium mixing bowl, combine confectioners' sugar with meringue powder.  Add water and vanilla extract and beat with a hand mixer on medium-high speed, until smooth. If you find that the icing is too thick, add a little water (a teaspoon at a time). If you find the icing too thin, add a little more sugar.
2. To colour the icing, add gel food colouring. You can also use liquid food colouring, but this will have a slight effect on the consistency of the icing. You may need to add a little more sugar to get the consistency right.
3. To decorate, use a no. 5 piping tip to outline the base colour on the sugar cookie, leaving about 1/8-inch from the edge, and fill in the inside. To get the icing completely to the edges, use the back of a spoon to spread the icing to the edge.
4. Allow the base to dry enough so that when you add icing on top, it won't smudge or blend into it. It doesn't have to be fully set, but set enough, about 20-30 minutes. Then, use a thin no. 1 round tip to write cute Valentine's Day messages on the cookies.  
5. Allow the icing to set completely before stacking up and storing to prevent smudging, a few hours up to one day.