Preheat your oven to 350F and spray an 8×8 pan with nonstick spray.
Pour the cookie mix into a large bowl and cut the butter into the mix using a pastry blender. Add the egg and continue to mix until the mixture is crumbly.
Divide the oatmeal mixture in half and press the first half into the bottom of the pan. Bake for 15 minutes.
In a medium bowl, stir the condensed milk, lemon zest, and lemon juice together. Once the oatmeal cookie mixture is done baking, pour the lemon mixture on top and spread evenly.
Loosely crumble the second half of the oatmeal cookie mixture on top. Bake for 25 minutes.
Allow pan to cool on cooling rack for 30 minutes. Then, transfer the pan to the cooling rack and allow it to cool for another 30 minutes.
