for the sugar cookies:

In the bowl of an electric mixer, beat together butter and sugar for about 2 minutes, until light and fluffy. Add the eggs and egg yolk, one at a time, mixing well in between each and scraping down the sides of the bowl as necessary. Add the vanilla and beat again until smooth.

In a medium bowl, whisk together the flour, salt, and baking powder. With the mixer on low speed, slowly add the dry ingredients until just combined. Wrap the dough in saran wrap and chill in the fridge for at least an hour (or overnight).

Preheat oven to 350 degrees and line two large baking sheets with parchment paper or Silpat mats. Roll out dough to about 1/4-1/8" thickness and use a circle cookie cutter to cut out the shapes. Place the cookies about 1" apart on prepared cookie sheets and bake for 8-10 minutes (shy on the undercooked side - they might look like they're not done but they'll cook a bit more on the cookie sheet and you don't want them crunchy). Allow cookies to cool on the pan for 5 minutes, then transfer to a wire rack to cool completely before decorating.

for the royal icing:

In the bowl of an electric mixer, mix together the powdered sugar, meringue powder, water, and vanilla until smooth (about 2 minutes). To achieve the moderate consistency icing, keep adding water 1 tbsp at a time until a ribbon of icing drizzled over the bowl disappears/"melts" back into the bowl of icing in about 20 seconds. It's somewhere between the consistency of outlining royal icing and flood icing if you've made royal icing before. Keep adding water until you're happy with the consistency, and make sure to beat well in between each addition.

Remove about 1/2 cup of the prepared icing to a separate bowl. Add yellow food coloring to the mixer and beat until evenly distributed. Add black food coloring to the smaller bowl of icing until you get a dark black color. Cover this bowl with damp paper towels or rags so it doesn't harden as you do the yellow layer first. Transfer the yellow icing to a plastic squeeze bottle or piping bag fit with a small round tip, and fill in the circles with yellow icing. Allow them to dry uncovered for at least 6-8 hours before adding the black.

Once the yellow icing is dry, add the candy heart eyes to each cookie. I used a tiny dot of leftover icing to secure each one. Then transfer the black icing to a squeeze bottle or piping bag with a small round tip and add the smiles. Allow this layer to dry again for another 6-8 hours, then store in an airtight container.