Make the cookie dough.
Combine cake mix, eggs, and oil in large mixing bowl, and beat well. Stir in Reese's Mini Pieces baking chips with a spatula. Cover and chill the dough for at least an hour, or up to overnight.
Preheat and prepare the pan.
Preheat oven to 350 degrees and line two cookie sheets with parchment paper for easy clean up. Scoop dough into ball just larger than a golf ball. Arrange on a cookie sheet, pressing down to flatten dough into a round disc. These cookies do not spread much.
Bake then cool.
Bake for 9-10 minutes depending on the size of your cookie, rotating the pan halfway through baking time. Cookies should be slightly underbaked for best results. Remove from oven and let cool on the cookie sheet 5 minutes then transfer to a wire rack to cool completely. Repeat with remaining dough. Makes 15-18 cookies.
Adapted from The Frugal Girls