In a large bowl, whisk cake mix, eggs, butter, and oil until just combined. (The dough will be quite thick and start to stick to the whisk.) Switch to a spatula and mix until a smooth dough forms. Fold in chocolate. Cover with plastic wrap and refrigerate until set, 5 to 10 minutes.
Arrange a rack in center of oven; preheat oven to 350°. Line 2 baking sheets with parchment, then grease parchment with cooking spray. Using a ¾-oz. scoop, scoop dough into golf ball-sized balls.
Pour crushed candies onto a plate. Roll tops of balls in candies. If baking immediately, transfer dough to prepared sheets, or store in an airtight container and refrigerate up to 4 days.
Gently press down on tops of balls with the palm of your hand to slightly flatten. Bake cookies 3 minutes. Remove pans and bang against a solid surface (such as the stove or a wooden cutting board) about 6 times. This will help the cookies spread and take on the desired texture. Continue to bake until no longer glossy on top but soft and moist inside, about 3 minutes more.
Let cool on sheets 5 to 10 minutes, then transfer to a wire rack and let cool completely.
