Brown butter cake layers with marshmallow fluff filling, rice krispie treats and brown butter frosting.
Preheat the oven to 350 degrees F. Prepare three 8-inch or four 6-inch round cake pans with non-stick spray, parchment paper on the bottom of the pan, and another coat of spray. Set aside.
Turn the mixer on low and add 1/3 of the flour mixture to the butter mixture and mix until fully incorporated. Then add half of the buttermilk and blend well, followed by another 1/3 of the flour mixture, the second half of the buttermilk and lastly, add the remaining flour mixture and mix until incorporated.
Evenly distribute the cake batter among the three 8-inch pans, about 16 ounces in each or 12 ounces in each 6-inch pan, and bake for 20 to 25 minutes, until toothpick comes out with just a few moist crumbs on it.
Meanwhile, heat the sugar, corn syrup, water and vanilla extract. Heat over medium heat until the sugar is dissolved and just simmering, about 5 minutes.
Spread a dollop of frosting on a cake board or cake plate to act as "glue" to hold your first cake layer in place.
Place the first cake layer, top side up, in the center of the cake board or cake plate. Apply a rim of the brown butter frosting around the edge of the cake. This will act as a barrier to the marshmallow fluff and will help to support the cake layers as you stack.
Add about one cup of the marshmallow fluff and spread evenly within the frosting barrier.
Sprinkle Rice Krispie Treats over the marshmallow fluff and pat into place. If you're not serving your cake right away, omit the Rice Krispie Treats in between the layers and serve on top of the cake slices as you serve to your guests.
Place the second cake layer, top side up, on the fillings. Repeat steps with the frosting barrier, marshmallow fluff and Rice Krispie Treats.
Place the final cake layer, top side down, so the smoothest and straightest side is on top.
Apply a thin layer of frosting around the entire cake. This is the crumb coat. Freeze the crumb coated cake for about 10 to 15 minutes to set the frosting and crumbs in place.
After the crumb coat is set, continue to frost the cake with the remaining frosting.
Use the remaining marshmallow fluff and Rice Krispie Treats to decorate the top of the cake.
*This will make enough to fill the layers and pipe some on top of the cake