Preheat your oven to 350°F. In a large mixing bowl, combine the spice cake mix, eggs, buttermilk, vegetable oil, and sour cream. Mix on medium speed just until the ingredients are fully blended. Add the vanilla extract, cinnamon, nutmeg, and brown sugar, then continue mixing until everything is well incorporated. Line a standard muffin pan with cupcake liners and fill each liner about two-thirds full with the batter. Bake the cupcakes at 350°F for 15 to 18 minutes, or until a toothpick inserted into the center comes out clean. Once baked, allow the cupcakes to cool in the pan for five minutes, then transfer them to a wire rack to cool completely before frosting.

To make the frosting, begin by placing the oatmeal cream pies in a food processor and pulsing them into fine crumbs. Set the crumbs aside. Allow the butter to soften at room temperature until it is slightly softened but not melted. Using a stand mixer fitted with the paddle attachment, beat the butter for several minutes until it becomes smooth and creamy. Add the jar of marshmallow fluff to the butter and continue beating until fully combined. Mix in the vanilla extract until evenly incorporated. Gradually add the powdered sugar to the mixture, about two cups at a time, mixing well after each addition. For a firmer frosting, use up to five cups of powdered sugar. Add the heavy whipping cream to the frosting and beat until it becomes smooth and creamy—this helps balance the sweetness and improve texture. Finally, mix in the cinnamon and a pinch of salt. Once blended, add the oatmeal cream pie crumbs and beat on low speed just until incorporated.