Preheat oven to 350°F.
Mix the oatmeal cookie mix: Stir 1 stick softened butter, 1 Tbsp. water, and 1 egg in a medium bowl. Stir in 17.5 oz. box oatmeal cookie mix until you have a stiff dough.
Press the dough flat into a greased 9×13″ baking dish and bake for 25-30 minutes or until baked through. Remove from the oven.
Let the oatmeal cookies cool completely, then crumble them. Add 2 cups of the crumbled cookie mix with 3 tablespoons of softened butter and 1 1/2 tablespoon brown sugar to a food processor, and process until you have a fine crumble.
Grease an 8×8″ baking dish and press the oatmeal crumble mix into the bottom of the pan.
In a large bowl, whisk together 3/4 cup granulated sugar, 1/2 cup brown sugar, 2 Tbsp. nonfat milk powder, 1/4 tsp. salt, 1/2 cup unsalted softened butter, 1/3 cup heavy cream, 1 tsp. vanilla extract, and 2 Tbsp. corn starch until well-combined.
Pour the pie filling mixture over the oatmeal crust. Bake for 30 minutes at 350°F.
Reduce heat down to 325°F and bake for an additional 20 minutes until the edges are set and the center is almost set. The center will be slightly jiggly.
Remove from the oven and let cool at room temperature. Refrigerate overnight before serving.
When ready to serve, sprinkle with powdered sugar, cut into square bars, and enjoy!