Preheat the oven to 350° F. Lightly spray a 9-inch by 13-inch pan with lecithin spray.

To prepare the Crumb Topping, toast the walnuts in the preheated oven for 7 to 10 minutes, stirring occasionally. Let the nuts cool. Then process in a food processor, using the pulsing action, until they are coarsely chopped. Use an electric mixer on low speed to mix the flour, fruit sweetener, cinnamon, and salt just until crumbly. Then stir in the toasted walnuts. Cut the butter into 1/2-inch pieces and add it to the nut mixture, keeping the mixture crumbly. Set aside.

To prepare the Coffee Cake batter, use an electric mixer on low speed to combine the flour, sweetener, cinnamon, and salt just until they resemble coarse cornmeal. With the mixer running, pour in the melted butter. Mix until a smooth batter is created. It is important to stop the mixer occasionally and scrape the bottom and sides of the mixing bowl to ensure even mixing and a lump-free batter. Remove 1 cup of this batter and add it to the Crumb Topping above, mixing with your fingers until it is evenly dispersed, though the mixture must remain crumbly.

Stir the baking powder and the baking soda into the rest of the batter. Add the eggs one at a time, beating well after each addition. Add the buttermilk gradually, then the vanilla, again scraping the bowl with a rubber spatula to ensure even mixing. Spread the Coffee Cake evenly in the prepared pan. Use your hands to evenly distribute the Crumb Topping over the top of the cake.

Bake the cake in the preheated oven for 45 to 50 minutes until it is golden brown and springs back when touched lightly in the center. Serve Coffee Cake hot from the oven.