• Preheat the oven to 325 degrees. Generously grease a Bundt pan with butter and dust with 1/4 cup of flour, shaking out any excess.
• In a large bowl, whisk together the flour, baking soda, salt, and cinnamon.
• To the bowl of a stand mixer, add the sugar, eggs, canola oil, orange juice, and vanilla extract and mix until well combined. Gradually add the flour mixture, beating until thoroughly incorporated.
• Gently fold the apples and walnuts into the batter with a rubber spatula. Pour the batter into the prepared Bundt pan and bake for 1 1/2 hours, or until a wooden pick inserted in the center comes out clean.

Sauce

• About 10 minutes before the cake is done, melt the butter in a large saucepan over medium high heat. Stir in the sugar, buttermilk, and baking soda. Bring the mixture to a boil and cook for 1 minute, stirring constantly.
• Remove the cake from the oven and set the pan on a wire rack. Poke holes in the cake with the end of a narrow wooden spoon or the prongs of a meat fork.
• Pour the hot sauce all over the cake in the pan. Let the cake sit in the pan for 1 hour before removing to a wire rack to cool completely.