Step 1In a medium bowl, whisk together the sugar, flour and cinnamon. Cut in the cold butter until the mixture becomes crumbly. Set aside.
Step 1Heat the oven to 350 degrees. Grease a 9-by-5-by-3-inch baking dish. Line the base of the dish with parchment paper, then grease the top of the parchment paper.
Step 2In the bowl of a stand mixer, or in a large bowl using an electric mixer, cream together the butter and sugar until light and fluffy, 3 to 5 minutes. Add the eggs, one at a time, beating well until fully incorporated. Beat in the buttermilk and vanilla.
Step 3In a medium bowl, whisk together the flour, baking soda and salt. With the mixer running, slowly beat in the dry ingredients just until fully combined. Stir in the diced apples and chopped walnuts by hand.
Step 4Pour the batter into the prepared dish, then sprinkle over the topping.
Step 5Place the dish in the oven and bake until puffed and a toothpick inserted comes out clean, about 55 minutes to an hour. Cool the dish on a rack for 15 minutes, then unmold and cool completely before slicing.