In a bowl, whisk together the flour, salt, and baking powder, and set aside.

In the bowl of a large stand mixer fitted with a paddle attachment, combine the butter and sugar, and mix on medium speed until fluffy and well-mixed, about 1-2 minutes.

Add the eggs one at a time, beating well after each addition, then add the vanilla extract and sour cream.

With the mixer running on low, gradually add the dry ingredients and mix until just incorporated. Stop the mixer when only a few streaks of flour remain and finish mixing by hand with a spatula.

Divide the dough in half and press each into a thin disc, wrapping well with plastic wrap. Refrigerate until firmer but not hard, about 30 minutes.

Preheat the oven to 350 degrees F. Place a disc of dough between two sheets of parchment or waxed paper, and roll out until 1/4-inch thick.

Use a 3-inch heart cookie cutter to cut out cookies and place them on a parchment-lined baking sheet. Combine scraps and re-roll the dough to cut more cookies. You should get about 24 large heart cookies.

(Optional) Put the sheets of cut-out cookies in the freezer for 10 minutes before baking to help the cookies hold their shape.

Bake the trays of cookies for 10-12 minutes until the edges are just starting to take on color and they have lost the raw shine in the middle. Let cool on the baking sheet for 5 minutes, then remove them to wire racks to cool completely before decorating.

Combine meringue powder and water in a bowl of a large stand mixer fitted with a paddle attachment. Mix on medium speed until foamy.

Stop the mixer and sift powdered sugar over the bowl. Beat on low speed until incorporated, scrape the bottom and sides of the bowl, then add the corn syrup and flavoring.

Turn the mixer speed to medium-high and mix for 5 minutes until the royal icing is shiny, stiff, bright white, and voluminous.

Separate out half of the icing to leave plain white, pressing cling wrap on top to avoid crust formation.

Divide the remaining icing into bowls for coloring, add a few drops of gel food coloring to each bowl and stir until evenly distributed.

For designs requiring thinner icing, add a small spoonful of water to each colored icing and stir until it reaches a consistency that takes about 15 seconds to fully disappear in the bowl.

Place each color in a piping bag fitted with a small round tip.

Work in small batches by flooding 2-3 cookies at a time with white frosting, then piping designs on top.

For swirled hearts, pipe dots of color on the outer edge and drag a toothpick through the dots.

For zigzag designs, pipe lines of alternating colors and drag a toothpick through vertically to create a zig-zag pattern.

For elegant rose cookies, pipe blobs of color and swirl together with a toothpick, adding leaves if desired.

For stiff rosettes, pipe small swirls from the center outward with stiff consistency frosting after base cookies have dried for at least an hour.

Let the royal icing set for at least 2 hours before serving. Stack cookies only after they have dried for 12 hours.