Preheat oven to 375F. Line 2 baking sheets with non-stick baking mats or parchment paper.
Place all ingredients in a large bowl. Beat with an electric mixer on low speed until combined (about 3-4 minutes).
1 box devil's food cake mix, 4 oz (125g) cream cheese, 1/2 cup (132g) smooth peanut butter, 1 large egg
Using a medium cookie scoop or tablespoon, drop rounded cookie dough onto prepared baking sheets. Flatten each ball in a criss-cross pattern using the tines of a fork dipped in sugar.
Bake for about 8 minutes or until edges of cookies are set. Cool on baking sheets for 2 minutes and remove to wire racks to cool completely.
Notes
Store leftover cookies in an airtight container at room temperature for up to 5 days.