In a small bowl, whisk together the flour, salt, and baking soda.

In a medium bowl, beat the butter and sugar together until light and fluffy.

Add the egg and vanilla and beat until combined.

Add the flour mixture and beat until well combined.

Transfer dough to a flat work surface and press into a rectangle about 1-inch thick. Wrap dough in plastic wrap and chill for 1 hour.

Preheat the oven to 350 F. Line two baking sheets with parchment paper.

Place dough on a lightly floured surface and roll out to about ¼ thickness. Cut into shapes using the triangle cookie cutter.

Place cut out cookies on prepared baking sheets and chill for 15 minutes.

Bake for 8-10 minutes or until the edges are light golden.

Let cookies cool for 2 minutes on the baking sheets before transferring to a wire rack to cool completely.

In a medium bowl, whisk together the powdered sugar and meringue powder.

Add ¼ cup of warm water and beat on low speed until well combined. Beat on high speed until glossy and the icing holds stiff peaks.

Stir in additional warm water, a tablespoon at a time, until icing creates loose ribbons when drizzled with a spoon. Aim to have the icing ribbons disappear back into the bowl within 10 seconds of being picked up with the spoon.

Divide the icing into three bowls, placing smaller amounts of icing into two bowls (for the two greens) and a larger amount in the third bowl (for the pinkish red). Cover the bowls with plastic wrap when not working with them.

Using the green gel food coloring, dye one of the smaller bowls dark green and the other smaller bowls light green. Dye the last bowl a pinkish red color using a combination of pink and red gel food coloring.

Transfer icing to piping bags fitted with round piping tips. If you only have one round piping tip, use a coupler so you can easily transfer the tip between bags.

Pipe and fill a small dark green rectangle around the back edge of the triangle cookies. This will be the watermelon rind. Pipe this section on all of the cookies before moving onto the light green.

Pipe and fill a small light green rectangle right below the dark green.This will be the second part of the watermelon rind. Pipe this section on all of the cookies before moving onto the pinkish red.

Pipe and fill a large triangle below the light green section, following the edge of the cookie. This will be the watermelon flesh.

Add a few Mickey shaped sprinkles to the watermelon flesh to make watermelon seeds.

Let dry completely before stacking or storing cookies.