In a large mixing bowl, whisk together the meringue powder and the bottled lemon juice vigorously for about 1 - 2 minutes. Stop when the meringue powder is completely dissolved and there's a layer of large, foamy bubbles on the surface.
¼ cup bottled lemon juice, 2½ TBS meringue powder
Add the powdered sugar. Using an electric hand mixer, mix on medium speed for about 2 minutes. The icing should be smooth and shiny. Be careful not to overmix, or you'll add too much air to the icing and it won't dry right.
½ lb powdered sugar
Drag a spoon through the icing and count how long it takes for the icing to come back together and be smooth again.This recipe makes about a 20-second consistency, meaning it takes about 20 seconds for it to come back together. This consistency is versatile and works for both outlining and flooding.Mix in lemon juice or water, drip by drip, to thin the icing out if needed. Color the icing (optional): Separate the icing into bowls. Add gel food coloring to the icing and mix until colored throughout.
gel food coloring
Transfer icing to piping bags and start to decorate your cookies.
Store in an airtight container on the counter for up to 2 days, or in the fridge for up to a week. Stir well before using.