Individual apple chestnut cakes with a walnut crumb

I have been making desserts with apples lately. There was the apple bread pudding, followed by the apple crumble pie, and the latest being, the individual apple chestnut cakes with a walnut crumb on the top. Again, just reiterating and repeating, I am one of the worst bakers in the world. Cakes and cookies are definitely not my forte, no scratch that, I am incapable of baking edible cookies and cakes. But I had one granny smith apple left over when I made the apple crumb pie. Again, browsing through the apple based desserts in food and wine magazine, I thought of making these individual cakes.

The recipe in question can be found at the following link.

http://www.foodandwine.com/recipes/apple-crumble-cakes-with-chestnuts
This recipe was for eight, and I narrowed it down to 4. I am using most of the text of the original recipe with my changes.
Preheat the oven to 350°. Grease 4 8-ounce ramekins and place on a baking tray.
Crumb
Toss the following together
• ¾ cups coarsely chopped walnuts which I pounded in the mortar
• 1/3 cup light brown sugar
Cake
Blend the following wet ingredients
• 1 large egg
• ½ cup milk
• ½ cup white sugar
• ¼ cup brown sugar
• ½ stick of butter
• 1 tspn vanilla extract
Chop the following
• 1 large Granny Smith apple—peeled and cut into 1/2-inch dice
• 3/4 cup roasted chestnuts, quartered
Whisk the following dry ingredients
• ½ teaspoon cinnamon
• ½ teaspoon baking powder
• ½ teaspoon baking soda
• 1 cups all-purpose flour
Directions
1. In a medium bowl, whisk together the flour, baking powder and baking soda. In a large bowl, blend the butter with the sugar, brown sugar and eggs. Gently stir in the flour mixture and milk until blended, then fold in the apples and chestnuts.
2. 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, and then serve with a scoop of ice cream.