For the topping
1/4 lb. (1/2 cup) unsalted butter
1/2 cup firmly packed light brown sugar
1/2 cup granulated sugar
1-1/4 tsp. ground cinnamon
4-1/2 oz. (1 cup) all-purpose flour
1/4 cup confectioners' sugar
4 oz. (1 cup) whole walnuts, coarsely chopped
For the cake
9 oz. (2 cups) all-purpose flour
1 tsp. baking powder
1 tsp. baking soda 3/4 tsp. ground cinnamon
1/2 tsp. table salt
3 oz. (6 Tbs.) unsalted butter, softened at room temperature
1-1/4 cups granulated sugar
2 large eggs, at room temperature
3/4 cup smooth homemade or store-bought applesauce
1 cup sour cream
To start, make the crunchy topping. Combine melted butter, brown sugar, regular sugar, and cinnamon. Then stir in flour, confectioners' sugar and chopped walnuts and mix until blended but still crumbly. Refrigerate the topping until you're ready to use it.
Next, make the cake. Combine flour, baking powder, baking soda, and cinnamon and set aside. Beat butter and sugar with an electric mixer until blended. Beat in eggs, one at a time, and then alternate beating in the flour mixture, applesauce, and sour cream. Don't overmix. Scrape the batter into a greased 9 x 13 inch baking pan.
Bake until the edges of the cake are slightly set and the rest is very jiggly. Scatter the topping over the came and bake until crumbs are golden brown, and the center springs back when gently pressed. Let cool on a rack for 20 minutes before cutting and serving.