Whisk together the flour, baking soda, and salt in a medium bowl.
Beat the softened butter and sugar in a large bowl for about 2 minutes until creamy.
Add the egg and vanilla extract. Mix until combined.
Add the green gel food coloring gradually, mixing until the color is even.
Add the dry ingredients in two additions and mix just until a soft dough forms.
Cover and refrigerate the dough for 30 minutes.
Preheat the oven to 350°F (175°C) and line baking sheets with parchment paper.
Scoop the dough into portions of about 1½ tablespoons, roll into balls, and space them about 2 inches apart.
Bake for 10–12 minutes, until the edges are set and the centers still look slightly soft.
Cool on the baking sheet for 5 minutes, then transfer to a wire rack.
Press a red heart sprinkle into each cookie while it is still slightly warm, if desired.
Once completely cool, decorate with black icing and yellow candy eyes if making Grinch faces.
Let the decorations set before serving or storing.