Place butter and sugar in the bowl of a large stand mixer, fitted with a paddle attachment OR in a large mixing bowl with a wooden spoon. Mix on medium speed or by hand until creamy, about 2 minutes. Add egg, vanilla, and food coloring, beating well until smooth.
In a separate medium bowl, combine flour, cocoa, baking soda, and salt. Fold into butter mixture until dough is just combined. Wrap dough in plastic wrap and chill in refrigerator at least 1 hour or up to 6 days.
When ready to bake, preheat oven to 350F and line two large sheet pans with parchment paper.
Scoop dough into 1-inch balls and roll in sanding sugar until evenly coated. Place dough balls 1 inch apart on prepared sheet pans.
Bake cookies at 350F 10-12 minutes until cookies are puffy and cracked on top. Allow cookies to cool on sheet pan 10 minutes then transfer to a cooling rack to cool completely. Enjoy!