Preheat the oven to 350°F. Grease a 9x13 baking dish and set aside.
In a medium mixing bowl, beat together the cake mixes, eggs, and vegetable oil. Beat with a hand (or stand) mixer until combined. The mixture will be thick and crumbly.
Remove half of the mixture and set aside. Press the remaining dough into the bottom of the prepared baking dish.
Bake for 15 minutes, then remove from the oven.
Meanwhile, prepare the lemon pie filling. In a medium saucepan, stir together sugar, cornstarch, and water. Add egg yolks and whisk in until mixture is combined.
Turn on the stovetop and bring the mixture to a simmer, cooking over low to medium low heat and whisking constantly.
Cook until mixture has thickened and comes to a low boil for at least 1 minute.
Remove the mixture from the heat and stir in lemon zest, lemon juice, and the butter. Whisk until combined.
Pour the lemon pie filling over the pre-baked bar base.
Sprinkle the reserved dough in clumps over the top of the lemon pie filling.
Return the baking dish to the oven and bake for 15-20 minutes longer, until the top of the bars is just beginning to brown.
Cool bars completely.
Whisk together powdered sugar and enough milk to make a sugar glaze. You can use lemon juice instead of the milk to add some additional lemon flavor. Drizzle over the top of the cooled lemon pie bars.
Cut into bars and serve with plates and forks.
Notes
For a thinner filling and sturdier cookie bar: use half of the lemon pie filling.
Instead of vanilla cake mix, you could use white, yellow, or lemon cake mix.
To make a lemon glaze: use lemon juice instead of the milk. If you want to keep the milk for the glaze, you can add 1-2 drops of lemon oil for an extra punch of flavor.
For a more tart lemon pie filling: use Eureka lemons
To make a sweeter, mellow filling: use Meyer lemons
If you want to use up the leftover egg whites, make my lemon meringue pie!
Store on the counter for 2-3 days or freeze for up to 1 month.