Combine the dry ingredients. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
Cream the butter and sugar. In a large bowl, beat the softened butter and 1¼ cups sugar for 2–3 minutes, until light and creamy.
Add the wet ingredients. Beat in the egg, egg yolk, and vanilla until smooth and combined.
Make the cookie dough. Gradually add the dry ingredients to the butter mixture. Mix just until a soft dough forms. Avoid overmixing.
Color the dough. Divide the dough evenly among three bowls. Add purple gel food coloring to one, green to another, and orange to the third. Mix each until the color is evenly distributed. Add more gel coloring as needed to get vibrant Halloween colors.
Prepare for baking. Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper. Place the remaining ⅓ cup sugar in a shallow bowl.
Shape and coat. Scoop about 1½ tablespoons of dough per cookie and roll into balls. Roll each ball in granulated sugar until evenly coated.
Bake. Arrange the cookies about 2 inches apart. Bake for 10–12 minutes, until the edges are set but the centers still look slightly soft.
Add the monster eyes. Immediately after removing the cookies from the oven, gently press edible candy eyes into the warm cookies. Use different sizes and numbers of eyes to give each monster a different look.
Cool. Let the cookies cool on the baking sheet for about 5 minutes before transferring them to a wire rack.