Make the topping
Melt the butter in a medium bowl in the microwave (or in a skillet and pour into a medium bowl). Add the brown sugar, granulated sugar, and the cinnamon and stir until blended. Add the flour, confectioners' sugar, and the walnuts, mixing with your fingers until it's well combined; the mixture should be crumbly but also clump together. Refrigerate until ready to use.
Make the cake
Heat the oven to 350°F. Butter the bottom and sides of a 9x13-inch baking pan. Line the bottom of the pan with parchment and butter the parchment.
In a medium bowl, whisk the flour, baking powder, baking soda, cinnamon, and salt to blend.
Combine the butter and sugar in a large bowl. Beat with an electric mixer on medium speed until combined (but not fluffy), scraping the bowl as needed, about 30 seconds. Add the eggs, one at a time, beating until combined and scraping the bowl as needed.
Add about one-third of the flour mixture, mixing on medium low until combined. Add the applesauce, mixing on medium low until incorporated and scraping the bowl as needed. Mix in another third of the flour mixture, then the sour cream, and then the remaining flour mixture, mixing after each addition until just incorporated. Don't overmix. Scrape the batter into the prepared pan and spread it evenly.
Bake until the edges of the cake are slightly set and the rest is very jiggly, about 15 minutes. Scatter the walnut topping evenly over the cake and bake until the crumbs are golden brown, the center of the cake springs back when lightly touched, and a toothpick inserted in the center has a few moist crumbs sticking to it, 30 to 35 minutes; check the cake early and if the crumbs are golden but the cake isn't fully baked, cover loosely with foil. Let cool on a rack for at least 20 minutes. Serve warm.
nutrition information (per serving):
based on sixteen servings, Calories
18, Fat Calories
160, Saturated Fat
5, Monounsaturated Fat
51, Polyunsaturated Fat
Photo: Scott Phillips