Preheat the oven to 350°F. Line two baking sheets with parchment paper and set aside.

In the bowl of a stand mixer, combine the softened butter and sugar.

Cream the butter and sugar together until smooth.

Add in the egg and vanilla and mix to combine.

Add in the flour and mix well, scraping the sides and bottom of the bowl often until a cohesive dough forms.

Transfer the dough to your work surface and knead well until smooth.

Lightly flour the work surface and use a rolling pin to roll out the dough to about ⅝-inch thickness.

Use a rectangular cookie cutter to cut out as many rectangles as you're able. Save the dough scraps and re-roll as many times as possible.

Carefully transfer the cut-out cookies to the prepared baking sheets, leaving about ½–1 inch between each cookie. They should not spread much.

Bake in the preheated oven for 11–12 minutes, just until the edges are slightly browned and the center is set. Do not overbake. Allow to cool completely before decorating.

In one bowl of a stand mixer, combine 2 cups of the powdered sugar, 1 teaspoon vanilla, and 2 tablespoons meringue powder. Mix in about ¼ cup of water and use the paddle attachment to beat well for about 2–3 minutes until thoroughly combined, scraping the sides often.

In a second bowl, combine the remaining 2 cups powdered sugar, 1 teaspoon vanilla, 2 tablespoons corn syrup, and about ¼ cup of water. Mix with the whisk attachment until thoroughly combined, scraping the sides often.

Transfer both icings into one bowl and mix together using the whisk attachment. Scrape the sides often to ensure it's thoroughly combined.

If needed, thin the consistency of the icing by adding water one tablespoon at a time until it flows like thick honey.

Add the green food coloring and mix well, scraping the sides and bottom often until evenly and deeply colored.

Transfer the green icing to a piping bag. When ready to decorate, cut a small opening in the tip.

Pipe around the outside edges of each cooled cookie, then fill in the center. If needed, use a toothpick to help spread the icing into the corners and pop any air bubbles.

Allow the decorated cookies to set for 12–24 hours for the icing to harden completely before proceeding.

Melt the white chocolate in a microwave-safe bowl in 30-second intervals, stirring between each, until smooth. Transfer to a piping bag and set aside to cool slightly.

Cut a few straws into 3-inch sections. Place a rubber band around them to keep them tight and upright in a small bowl.

Cut a small hole in the end of the piping bag and pipe the melted white chocolate into each straw. Allow to set completely.

To remove the white chocolate from the straws, either carefully unwrap paper straws, or cut open plastic straws lengthwise. Handle the chalk gently.

Once the icing is completely hardened, very carefully write your desired message or decoration on each cookie using the white chocolate chalk. The white chocolate may crumble a bit — this is intentional and mimics real chalk dust on a chalkboard. Do not press too hard to prevent the chalk from breaking or indenting the icing. Instead, take several light passes over the same area to build up the color.