In a large bowl, combine sliced apples, 1/4 cup of sugar, lemon juice, and cinnamon. Toss to coat the apples evenly.
Transfer the apple mixture to a greased 9x13-inch baking dish.
In another bowl, prepare the snickerdoodle cookie mix according to package instructions, substituting melted butter for the water, and add 1 teaspoon of vanilla extract.
Spread the cookie dough over the apple mixture evenly.
Sprinkle the top with the remaining 1/4 cup of sugar.
Bake in the preheated oven for 45 minutes or until the top is golden and the apples are tender.
Allow to cool slightly before serving.
Notes
Best served warm with a scoop of vanilla ice cream.