Preheat oven to 375. Use butter to coat a medium-sized shallow casserole dish.
Peel the apples and then chop them into ½-inch pieces. Place the apples in a large bowl. Add the lemon juice and gently stir to coat the apples. Add the vanilla, cinnamon, brown sugar, and cornstarch - gently mixing with a spatula until the apples are coated.
Pour the apples into the buttered casserole baking dish.
Stir together the dry ingredients for the topping in a large bowl. Add the butter. Use your fingers to smash the butter, mixing it into the dry ingredients to create a crumble.
Sprinkle half of the topping over the apples in the casserole dish. Gently mix the topping into the apples.
Sprinkle the remaining topping over the apples.
Bake for 35-40 minutes or until the top is golden brown (and bubbly!). Keep an eye on this dish because all ovens cook differently!
Allow the Apple Crisp to settle for at least 10 minutes before enjoying!
