Preheat the oven to 350°F. Grease a 9×13-inch baking pan and set it aside.

In a large bowl, combine 1 box yellow cake mix, 1 (21-oz) can apple pie filling, 1 tsp vanilla extract, ½ tsp cinnamon, 2 large eggs, and ¼ cup milk. Use an electric mixer on low to medium speed (or a sturdy spoon) and mix just until the ingredients are combined and there are no large dry pockets of cake mix.

Pour and spread the batter evenly into the prepared 9×13 pan.

In a separate large bowl (or the bowl of an electric mixer), combine ⅔ cup granulated sugar, ⅔ cup packed brown sugar, ½ tsp salt, 1½ tsp cinnamon, 1½ cups melted butter (cooled slightly), and 3½ cups all-purpose flour.

Mix the crumb mixture until it forms coarse, sandy crumbs. Use the mixer on low or mix with a spoon/flex spatula—stop when the mixture is crumbly and evenly moistened, not a smooth dough.

Sprinkle the crumb mixture evenly over the batter, starting at the edges and working toward the center to fully cover the surface.

Bake in the preheated oven for 35 to 45 minutes, until the top is golden and the center is set (a toothpick inserted near the center should come out with moist crumbs but not wet batter).

Remove from the oven and let the cake cool in the pan for at least 15 minutes before slicing and serving.