Preheat the oven to 350°F. Prepare three 6-inch cake pans or two 8-inch cake pans by spraying the sides with baking spray and fitting a wax or parchment paper circle to the bottom of the pans. Alternatively, you can grease and lightly flour the pans.

Using a food processor, grind the oatmeal into fine crumbs and place them into a medium bowl. Add the cake flour, baking powder, baking soda, cinnamon, and salt. Whisk to combine and set aside.

In the bowl of your stand mixer (a handheld mixer works fine too!), cream the butter on high until creamy, about 2 minutes. Add the vegetable oil, granulated sugar, and brown sugar and continue mixing on medium-high for another 2 minutes. Scrape down the bowl and paddle.

Turn the mixer to low speed and mix in the eggs one at a time. Add the sour cream and vanilla, then turn the mixer to high speed and beat for 1 minute.

With the mixer on low speed, add in the dry ingredients and mix until just combined, then add the milk in a slow stream and mix until just incorporated. Scrape down the sides and bottom of the bowl and give it a few stirs by hand to make sure there are no large lumps. The batter will be thick.

Divide the batter evenly between the prepared cake pans and bake for 30-35 minutes. They're done when they spring back to the touch and a toothpick inserted into the middle comes out clean. Let the cakes cool in the pan for five minutes before turning them out onto a wire rack for an additional few hours of cooling. Make sure they're entirely room temperature before applying any frosting.

In the bowl of a stand mixer with the paddle attachment or using a hand mixer, whip the butter on high speed until smooth and creamy, about 5 minutes. Turn the mixer to low speed and add the powdered sugar. Mix until fully combined, about 2 minutes.

Add the Jet-Puffed Marshmallow Creme, vanilla, and salt. Continue to mix on low speed until combined and uniform, 1-2 minutes.

With a hand mixer or paddle attachment on your stand mixer, cream the butter on medium-high until it’s creamy and light (almost white) in color. About 7 minutes.

Turn the mixer to low speed and add the sifted powdered sugar one cup at a time, scraping down after each addition and making sure each cup is fully incorporated before adding the next one.

Add the vanilla, milk, and salt and mix on low for another minute until fully incorporated, 1-2 minutes.

Once the oatmeal brown sugar cake layers have cooled completely, level and torte each layer to your desired height. Add about 1 Cup of vanilla buttercream into a piping bag and snip off ½ inch opening.

Place the first cake layer on a turntable and pipe a circle of vanilla buttercream around the edge of the cake layer. Fill in the center with half of the marshmallow cream filling. Here's a detailed tutorial on this filling method in case this is your first time working with a soft filling.

Place the next cake layer on top and repeat the process of piping the vanilla buttercream and filling in the center with the rest of the marshmallow cream filling. Place the final layer on top, upside down so that the bottom of the cake layer is the top of the cake. Crumb coat the cake with vanilla buttercream and place it in the refrigerator for at least 20 minutes to let everything set up.

To create the look pictured, use an icing smoother to create a semi-naked frosting finish with vanilla buttercream, then press the crumbled Oatmeal Creme Pies onto the bottom third of the cake. Place the cake into the refrigerator for about 10 minutes to let the frosting set while you add the rest of the vanilla buttercream into a piping bag fitted with Wilton Tip 4B. Pipe swirls around the top edge of the cake, leaving about ¼ inch between each swirl, then place a quartered section of Oatmeal Creme Pie in between each swirl.