Mix the chopped apples, granulated sugar, flour, cinnamon, and vanilla in a small bowl.
Transfer the apple mixture to a 10-ounce ramekin.
Make the Topping
In a separate small bowl, stir together the flour, oats, brown sugar, and cinnamon. Cut the softened butter into small pieces and work it into the dry ingredients with a fork or your fingers until the mixture clumps together.
Scatter the topping evenly over the apples. Place the ramekin on a small rimmed baking sheet.
Bake for 30 to 35 minutes, until the apples are soft and the topping is golden brown.
Video
Notes
Use softened butter, not melted. Softened butter should hold its shape but give easily when you press it with your finger. If it's shiny or looks like it's starting to melt, it's too warm. Properly softened butter blends into the dry ingredients in small pieces, which is what creates those distinct crispy clusters in the topping. Melted butter makes the topping dense and soggy.Use a 10-ounce ramekin. This is the right size for an apple crisp in a ramekin. A smaller dish will overflow.Cover with foil if the topping browns too quickly. Lay a piece of foil loosely over the ramekin for the last 10 minutes of baking if necessary.Place the ramekin on a baking sheet. The apple juices can bubble over during baking. A rimmed baking sheet catches any drips and keeps your oven clean.If doubling this recipe, bake in either a 4x6-inch or a 5x5-inch baking dish, or divide the mixture between two 10-ounce ramekins.