Make your faux egg. Mix the tapioca starch and water together in a medium size bowl. Set aside.
Mix the Dry ingredients together. Mix the boxed cake mix and baking powder together in a large bowl, breaking up any lumps. Set aside.
Mix the wet ingredients together. Whisk vegetable oil together with the non-dairy yogurt until well combined. Add in the tapioca flour/water mixture and vanilla extract. Whisk until all well incorporated.
Combine the wet and dry ingredients together. Add the wet mixture to the dry and mix with a large spoon until just incorporated. Fold in the sprinkles and crushed Oreos.
Chill the cookie dough. Cover the dough tightly with plastic wrap and refrigerate for at least 2 hours or overnight. Do not skip this step. The cookies will be much tastier and the dough easier to work with.
Preheat Oven. Once the dough is chilled, preheat the oven to 350 degrees and line 2 large cookie sheets with parchment paper or a non stick baking mats.
Roll the dough into balls. Scoop rounded spoonfuls on the prepared baking sheets (about 1.5 tablespoons per cookie). Make sure to keep the dough chilled if you are working in batches.
Bake. Bake for 10-12 minutes, or until the edges begin to set. The cookies will appear very soft, but they will set as they cool.
Cool. Leave the cookies on the baking sheet for an additional 4-5 minutes before transferring to a wire rack to cool.