PREHEAT the oven to 350°F (175°C) and line a flat cookie sheet with parchment paper. (See note #1.)
MELT the butter in the microwave for 45 seconds. Cool for a couple of minutes.
COMBINE the strawberry cake mix, pudding mix, melted butter, egg, and strawberry yogurt into a large mixing bowl. Stir with a large spoon until the dough comes together.
CHILL the cookie dough in the refrigerator for 30 minutes. (See note #2.)
SIFT the powdered sugar and cornstarch together in a small bowl. Place the granulated sugar in a separate bowl.
SCOOP the cookie dough balls with a medium scoop (1 tablespoon size is ideal). Roll the dough ball in your hands to make it smooth.
ROLL the balls first in a small bowl of granulated sugar until well coated.
DOUBLE ROLL the coated cookie dough balls in the powdered sugar mixture.
PLACE the cookie dough balls about two inches apart on the lined baking sheet.
SPRINKLE additional powdered sugar on top of the cookies so no dough is visible (optional).
BAKE for 9-11 minutes, or until the edges of the cookies are golden brown. The centers might seem slightly underdone, but they will continue to cook on the baking sheet after you remove them from the oven.
REMOVE the cookie sheet from the oven and turn the oven off.
PLACE the cookie sheet on a wire rack and cool for 10 minutes.
TRANSFER the cookies from the cookie sheet to the cooling rack. Cool them completely.