Prepare the Oven and Pan: Preheat the oven to 350°F (175°C). Line a muffin pan with cupcake liners, preparing enough for 24 cupcakes.
Mix the Cake Batter: Combine the spice cake mix, eggs, buttermilk, vegetable oil, and sour cream in a mixing bowl. Beat with an electric mixer on medium speed until the ingredients are just combined.
Add the Spices and Flavorings: Add the vanilla extract, cinnamon, nutmeg, and light brown sugar to the batter. Continue mixing until everything is evenly incorporated.
Fill the Cupcake Liners: Divide the batter among the prepared cupcake liners, filling each approximately two-thirds full to allow room for the cupcakes to rise.
Bake the Cupcakes: Place the muffin pan in the preheated oven and bake for 15–18 minutes. Check for doneness by inserting a toothpick into the center of a cupcake. If it comes out clean, the cupcakes are ready.
Cool the Cupcakes: Let the cupcakes rest in the pan for 5 minutes before transferring them to a wire rack. Allow them to cool completely before adding the frosting.
Crush the Oatmeal Cream Pies: Place the seven oatmeal cream pies in a food processor and process until they become fine crumbs. Set the crumbs aside for the frosting.
Cream the Butter: Place the softened, room-temperature butter in the bowl of a stand mixer fitted with the paddle attachment. Beat for several minutes until the butter is smooth and creamy. Make sure the butter is softened but not melted.
Blend in the Marshmallow Fluff: Add the entire 7-ounce jar of marshmallow fluff to the creamed butter. Beat until the mixture is smooth and well combined.
Add the Vanilla Extract: Pour the vanilla extract into the butter and marshmallow mixture. Mix until evenly incorporated.
Incorporate the Powdered Sugar: Add the powdered sugar in batches of approximately 2 cups at a time, mixing well after each addition. Use 4–5 cups in total, depending on the desired consistency. The original recipe uses 5 cups for a firmer frosting. Mix in the Heavy Cream: Add the heavy whipping cream and beat until it is fully incorporated into the frosting. This helps create a creamier texture and balances the sweetness of the powdered sugar and marshmallow fluff.
Season the Frosting: Add the ground cinnamon and a pinch of salt. Mix until both ingredients are evenly distributed throughout the frosting.
Fold in the Cookie Crumbs: Add the crushed oatmeal cream pie crumbs to the frosting. Beat on low speed just until the crumbs are incorporated.
Frost the Cooled Cupcakes: Once the cupcakes have cooled completely, spread or pipe the prepared frosting over them. Serve the finished cupcakes when ready.
