Preheat your oven to 350 degrees Fahrenheit and line 2 to 3 cookie sheets with parchment paper. This prevents the cookies from sticking and makes for easy cleanup.
In a medium bowl, whisk together the all-purpose flour, salt, and baking powder. This step is crucial as it ensures that the leavening agent is evenly distributed throughout the flour.
In another medium bowl, beat together the softened unsalted butter and granulated sugar for 2 to 3 minutes until the mixture is light and fluffy. The lightness of the mixture means you’ve incorporated enough air, which is key for the cookie texture.
Add the egg yolks, sour cream, and vanilla extract to the butter-sugar mixture and mix until just combined. Be careful not to overmix, as this could lead to tough cookies.
Gradually add the dry ingredients to the wet ingredients and mix until just barely combined. You want to see a few streaks of flour; it’s okay if the mixture isn’t fully homogeneous at this point.
This recipe doesn’t require chilling time, so you can go ahead and split the dough into 2 to 3 pieces. Roll each piece out on a lightly floured surface until it’s about ¼ inch thick.
Use a heart-shaped cookie cutter to cut out your cookies. Gather the scraps together, press them into a dough ball, and let them rest for about 5 minutes before rolling them out again to cut more cookies.
Place the cookies on the lined cookie sheets, leaving about 1 ½ inches between each cookie. This spacing allows them to spread without touching.
Bake in the preheated oven for 6 to 8 minutes, rotating the pan at the 4-minute mark to ensure even baking. The edges should be just set, and the tops will remain very light in color.
Let the cookies cool on the pan for 2 minutes to allow their shape to set before transferring them to a cooling rack. Once cooled, you can frost them or store them in an airtight container until you're ready to frost.
For the icing, using an electric mixer with the whisk attachment, mix the powdered sugar and meringue powder together on low. Gradually add 4 to 5 tablespoons of water and mix on high speed for about 1 minute. You want the icing to be pretty stiff.
Adjust the consistency by adding additional water one tablespoon at a time until it reaches the consistency of soft serve ice cream. Reserve about ⅓ cup of the icing in a separate bowl, covering tightly with plastic wrap for later use.
With the remaining icing, thin it out by adding water 1 to 2 teaspoons at a time until it drips back into the bowl and takes about 10 seconds to melt back into the icing. This will be used for flooding your cookies.
Separate the icing into small bowls and tint with gel food coloring to your desired colors. Use the reserved icing to create lettering by coloring it red or black.
To ice the cookies, fill your piping bags with the colored icing and snip the ends to create a small hole. Lightly trace the outer edge of the heart with the icing, using firm pressure to fill in the center. If there are any holes, use a toothpick to swirl the icing around.
Allow the icing to set for at least an hour until a crust forms. Once set, pipe your chosen words onto the cookies with the reserved icing. Let them sit uncovered at room temperature to fully dry, ideally overnight.
Once the icing is dry, your cookies can be stacked and stored in an airtight container for up to 7 days.
