Preheat oven to 350ºF. Grease the bottom and sides of an 8 or 9-inch pie tin.

In a medium bowl, add the cake mix, egg yolk and water. Using a fork or your fingertips, stir until combined and crumbly (it should clump when squeezed). If mixture is too dry, stir in additional water 1 teaspoon at a time.

Press cake mix mixture into the bottom and halfway up the sides of the prepared pie tin. Use a fork to poke lots of holes in the bottom and center (it puffs up as it bakes).

Bake until crust is deep golden and set, 9–10 minutes.

Let pie cool completely before filling.