Cinnamon Apple Walnut Coffee Cake

For the topping:

1/2 cup (about 2 ounces) finely chopped walnuts

1/4 cup granulated sugar

1 1/2 teaspoons ground cinnamon

For the cake:

1 1/2 cups cake flour

1/2 teaspoon baking powder

1/4 teaspoon baking soda

1/4 teaspoon salt

1 large egg

1 large egg yolk

1 cup granulated sugar

1/2 cup canola or corn oil, plus additional for the pan

1 teaspoon vanilla extract

1/2 cup sour cream

1 cup grated apple (2 medium apples, peeled)

Confectioners' sugar for dusting the top

For the topping: In a small bowl, combine the walnuts, sugar and cinnamon; set aside.

For the cake: Preheat the oven to 350 degrees. Lightly oil an 8-inch baking pan or aluminum foil pan.

In a medium bowl, combine the flour, baking powder, baking soda and salt. Set aside.

In a large bowl with an electric mixer on medium speed, beat the egg, egg yolk and sugar until fluffy and lightened in color, about 2 minutes. Reduce the speed to low, slowly add the oil and vanilla and mix just until incorporated. Slowly add the flour mixture and mix just until incorporated. Add the sour cream and mix just until blended and no white streaks remain. Using a spoon, stir in the grated apple. Scrape the batter into the prepared pan, using a spatula to smooth the top. Sprinkle half of the reserved topping mixture evenly over the batter. Dip a knife about halfway into the batter and swirl. Sprinkle the remaining nut mixture evenly over the batter.

Bake the cake in the preheated oven for about 50 minutes, until a toothpick inserted in the center of the cake comes out clean. Transfer the pan to a wire rack to cool completely. Dust the top with confectioners' sugar. Leave the cake in the pan for shipping. Or, cut the cake into squares and serve.