Wash and dry the apples. Using an apple corer or a paring knife, remove the core from each apple, leaving the bottom intact to create a cavity for the filling.
In a medium bowl, stir together the brown sugar, cinnamon, softened butter, and chopped walnuts until well combined.
Spoon the filling evenly into the center of each apple.
Arrange the apples in a baking dish and pour the water into the bottom of the dish around the apples.
Bake for 40-45 minutes, or until the apples are tender when pierced with a knife but still hold their shape.
Let the apples cool for 5-10 minutes.
Spoon the pan juices over the apples and serve warm with a scoop of vanilla ice cream, if desired.
Notes
Firmer apples such as Honeycrisp, Gala, Fuji, or Pink Lady work best because they hold their shape while baking.
Baking time may vary depending on the size and variety of the apples.
Leftovers can be stored in an airtight container in the refrigerator for up to 3 days and reheated in the microwave or oven before serving.