How to Make It
Preheat the oven to 350°. Place eight 8-ounce ramekins on a large rimmed baking sheet. In a bowl, toss the walnuts with the light brown sugar and cinnamon.
In a medium bowl, whisk together the flour, baking powder and baking soda. In a large bowl, blend the butter with the turbinado sugar, dark brown sugar and eggs. Gently stir in the flour mixture and buttermilk until blended, then fold in the apples and chestnuts.
Spoon the batter into the ramekins and sprinkle the walnut mixture on top. Bake for about 40 minutes, until the topping is browned and a cake tester inserted in the center of the cakes comes out clean. Let cool for 5 to 10 minutes, then serve with a scoop of ice cream.