Whisk together 3 1/2 cups (454g) of powdered sugar and 3 Tbsp (28g) of meringue powder in a large bowl or the bowl of a stand mixer.

Add 1/4 cup (60g) water and 2 Tbsp vanilla extract (24g). Mix on a low speed with a whisk or paddle attachment at first to incorporate the liquid, then increase to a medium speed and mix until stiff peaks form. This usually takes me about 3 minutes with a hand mixer or 2 minutes with a stand mixer. Scrape around the sides and bottom of the bowl with a rubber spatula to ensure everything is properly mixed together.

The fat in the cocoa powder can affect how well the icing whips up, so I like to add it last. Mix in the cocoa powder on a low speed until fully incorporated - I usually use 2 Tbsp, but you can adjust the color of the icing by using as little as 1 tsp of cocoa powder, all the way up to 3 Tbsp of cocoa powder. You can also use dark cocoa powder to deepen the color.

Then it's time to thin the icing to whatever consistencies you need. I usually add additional water 1 Tbsp at a time until a soft peak consistency is reached for outlines and piped details. I use about 2 Tbsp of water, but it can vary from batch to batch. To test for soft peak consistency, dip the spatula into the icing and focus on the way it curls at the end of the spatula. It should curl back onto itself but still be able to keep its shape. If it's too stiff, add a tiny bit more water. If it's too thin, add a bit more powdered sugar. You should have roughly 20 ounces or 2 1/2 cups of icing.

For flooding, I add additional water, 1 tsp at a time, to the remaining icing until the icing reaches flood consistency. To test for flood consistency, the icing should be able to run off the spatula in a thick stream, and you should be able to draw a figure 8 with the run-off icing. It should settle back into the bowl of icing within 8 seconds. Once the icing is thinned to this stage, pour it into a piping bag, and seal the top. I like to split the icing between a few bags so that it's easier to pipe with. If you overfill one giant bag, it can make decorating the cookies a lot harder on your hands. Set aside.

When you're ready to decorate cookies with. this icing, cut the tips of the piping bags to create a very small opening (1-2 millimeters). You can always cut the opening wider, but you can't make it smaller once you cut it. I find it's a lot easier to control the icing as you decorate the cookies if you have a small opening.

Outline each cookie with the soft peak consistency icing and let them dry for 5-10 minutes.

Then flood the cookies with the flood-consistency icing, one at a time. Then decorate as desired!

Let the cookies dry for a few hours or overnight until the icing is firm to the touch. These cookies can be stored in an airtight container for up to two weeks at room temperature or up to 3 months in the freezer.