For the apples:

Place chopped apples into a bowl of cold lemon water to prevent browning. Soak for 5 to 10 minutes, then drain and pat dry with paper towels. Heat butter in large non-stick skillet over medium-high heat until butter is sizzling hot. Add apples and saute until just beginning to brown, about 5 minutes. Add sugar, salt, cinnamon and nutmeg (if using), toss to coat, and saute until sugar has melted and starts to caramelize and turn golden brown, about 3 minutes longer. Set aside to cool.

For the cake:

Place oven rack in center position and heat oven to 350 degrees F (325 degrees for glass baking pans). Generously butter or grease two 8-inch square baking pans, or a 9- by 13-inch baking pan; set aside.

In a medium bowl, whisk together flour, baking powder, baking soda, cinnamon and salt. In a large bowl, beat butter until smooth, about 1 minute. Gradually add sugar and beat until light and fluffy, about 4 minutes. Add egg yolks (or eggs), one at a time, beating after each addition. Add vanilla and beat to combine. Add dry ingredients, 1/2 cup at a time, alternately with sour cream, beginning and ending with dry ingredients, mixing until just blended after each addition, scraping down sides of bowl with spatula as needed. Stir in cooled apple mixture. Pour batter into prepared pans and spread evely.

For the topping:

In a small bowl, combine brown sugar and cinnamon. Using a pastry blender, cut in butter until mixture is crumbly. Stir in nuts, and sprinkle topping evenly over batter.

Bake cake until topping is golden brown and a toothpick inserted near the center comes out clean, about 30 to 35 minutes for 8-inch squares, or 35 to 40 minutes for a 9- by 13-inch pan. When done, remove from oven and let cool on a wire rack for at least 15 minutes. Serve warm or at room temperature, with sweetened whipped cream or vanilla ice cream, if desired.

Wrap in plastic and store at room temperature for up to 2 days or in the freezer for up to 3 months.

Makes two 8-inch square coffee cakes (6 to 9 servings each), or one 9- by 13-inch coffee cake (about 15 servings).

Aluminum bakeware tip:

When baking and storing a cake containing acidic ingredients such as sour cream in an aluminum baking pan, line the bottom with parchment paper to prevent the juices from reacting with the aluminum and creating a metallic flavor.